Chapter IV: Introduction (2)

3. The =articular processes= are small. The anterior pair are in fact represented only by two oval facets on the anterior part of the laminæ which face almost directly upward. The posterior pair spring from the base of the spinous process; their facets face almost directly downward.

4. The =transverse processes= are short, thick, and tuberous at the free end. Each has a =facet= (Fovea transversaria) for articulation with the tuberosity of the rib which has the same serial number.

5. The =spinous process= is large, narrow, and slopes upward and backward. The anterior border is thin, the posterior wider and furrowed. The summit is expanded and rough.

The =first thoracic vertebra= is easily recognized by the following specific characters: The body is wide and flattened dorso-ventrally. In front it has a head like the cervical vertebræ, and behind a cavity somewhat deeper than any other thoracic vertebra. Two =costal facets= are found on either side, and a well-marked spine ventrally. The =arch= is large and strong, and has large notches. The =articular processes= are much larger than those of other thoracic vertebræ, and resemble a good deal those of the seventh cervical in form. The =transverse processes= are short and thick, and each has on its ventral aspect a large concave facet for articulation with the tubercle of the first rib. The spinous process is curved backward and tapers to a point. Its length is usually about three or four inches (ca. 8 to 10 cm.). It may be mistaken at first glance for the last cervical, but is promptly identified by the three costal facets on each side and the length of the spine.

The =last thoracic vertebra= is distinguished by the absence of the posterior pair of costal facets, and the confluence of the anterior pair with those on the transverse processes.

FIG. 14.—THIRD THORACIC VERTEBRÆ AND UPPER PART OF RIB OF HORSE,
ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 15.—LOWER PART OF NINTH THORACIC VERTEBRA OF HORSE, POSTERIOR
VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The serial position of others may be determined at least approximately by the following data: (1) The bodies gradually diminish in length and width to the middle of the region and then increase slightly. Their costal facets become smaller and less concave from first to last. The ventral crest is distinct on three or four vertebræ at either end of the region. (2) The transverse processes diminish in size and are placed lower down as they are traced backward. Their costal facets become smaller and lower in position; on the last (and sometimes on its predecessor also) it fuses with the costal facet of the body. The upper non-articular part of the process gradually becomes more sharply defined, and in the last four or five separates to form the =mammillary process=. (3) The spinous processes increase in length to the third and fourth, and then gradually diminish to the fifteenth, beyond which they have about the same length. The backward inclination is most pronounced in the second, the sixteenth is vertical, and the last two are directed a little forward. The longest spines (_i. e._, those of the withers) are the thickest and have expanded summits which remain more or less cartilaginous; the others are more plate-like, and are surmounted by a thick lip. The second spine is more than twice as large as the first. The summits of the fourth and fifth usually form the highest point of the withers.

=Development.=—There are =six= or =seven centers=, three for the body, two for the arch, and one for the spinous process; some of the latter have an additional center for the summit.

THE LUMBAR VERTEBRÆ

The =lumbar vertebræ= (Vertebræ lumbales) are usually six in number in the horse. They are characterized by the size and form of their transverse processes.

The =bodies= of the first three are triangular on cross-section, and present a distinct ventral crest. From the fourth backward they become wider and flatter and the ventral crest fades out.

FIG. 16.—FIRST THORACIC VERTEBRA AND UPPER PART OF FIRST RIB OF HORSE,
ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =arches= of the first two or three are about equal in size and similar to that of the last thoracic; from the fourth they decrease noticeably in breadth and height.

FIG. 17.—LAST TWO THORACIC AND FIRST LUMBAR VERTEBRÆ OF HORSE, LATERAL
VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =anterior articular processes= are fused with the mammillary processes, and present superiorly concave surfaces for articulation with the posterior pair of the preceding vertebra. The =posterior articular processes= project distinctly from the arch at the base of the spinous process, and have ventrally convex articular surfaces, which fit into the grooved surfaces of the anterior pair of the next vertebra.

The =transverse processes= are large plates, flattened dorso-ventrally, which project outward and usually curve slightly downward; their length increases to the third and fourth, and then diminishes to the last, which is the shortest. The first one or two usually curve somewhat backward, the last two decidedly forward. Those of the fifth have an oval concave facet on the inner part of the posterior border for articulation with the sixth process; the latter has a corresponding convex facet on the anterior border, and a larger concave surface on the posterior border for articulation with the wing of the sacrum. Sometimes the fifth process has a small surface for articulation with the fourth. The inner part of the sixth process is thick, the outer part thinner, narrower, and curved forward. The inner part of the fifth is also somewhat thickened.

The =spinous processes= resemble those of the last two thoracic vertebræ. They are usually about equal in height, but minor differences are common, and the width diminishes in the last three.

=Development.=—This is similar to that of the thoracic vertebræ. The extremities of the transverse processes remain cartilaginous for some time after ossification is otherwise complete.

FIG. 18.—SECOND LUMBAR VERTEBRA OF HORSE, ANTERIOR VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The transverse processes of this region are considered equivalent to
the proper transverse process + the costal element, and hence the
distinctive term processus lateralis (s. costarius) has been proposed.
The occurrence of a lumbar rib in connection with the transverse
process of the first lumbar is not rare. Reduction of the number to
five has been observed frequently, and may or may not be compensated
by an additional thoracic vertebra. This variation is not more common
in certain races as Sanson and others have maintained. Very few cases
are recorded of seven lumbar vertebræ—especially with the normal
thoracic number. An anomalous vertebra with mixed thoracic and lumbar
characters sometimes occurs at the junction of the two regions.

THE SACRUM

The =sacrum= (Os sacrum) is formed by the fusion of five vertebræ usually, and is conveniently described as a single bone. It is triangular in form and is wedged in between the ilia, with which it articulates very firmly on each side. Its long axis is gently curved, and slightly oblique, so that the posterior end is a little higher than the anterior. It presents two surfaces, two borders, a base, and an apex.

The =dorsal surface= presents centrally the five sacral spines, which are directed upward and backward, and have (with the exception of the first) tuberous summits which are sometimes bifid.

The first spine is relatively thin and narrow, and is not so high as
the internal angle of the ilium. The second is the longest, and the
height diminishes rapidly to the last. The bases of the spines are
often fused in old subjects.

On either side of the spines is a groove, in which are the four =dorsal sacral foramina= (Foramina sacralia dorsalia); the dorsal branches of the sacral nerves emerge through them.

The =ventral= or =pelvic surface= (Facies pelvina) is concave in its length, wide in front, narrow behind. It is marked by four more or less distinct =transverse lines= (Linæ transversæ), which indicate the demarcation of the bodies of the vertebræ. At the ends of these lines are the =ventral sacral foramina= (Foramina sacralia ventralia), which are larger than the dorsal series and diminish in size from first to last; they transmit the ventral divisions of the sacral nerves.

The dorsal and ventral foramina communicate with the sacral canal and are together equivalent to the usual intervertebral foramina.

FIG. 19.—SACRUM OF HORSE, DORSO-LATERAL VIEW.

_C_, Body of first sacral vertebra; _A_, arch of first vertebra; _Cs_,
sacral canal; _P.s._ 1–5, sacral spines; _P.t._, wings of sacrum; 1,
1′, surfaces for articulation with transverse processes of last
lumbar vertebra; _F.a._, auricular surface; 2, 2′, articular
processes; 3–6, dorsal sacral foramina; 7, interarcuate space; 8,
lateral border; _h_, apex. (Struska, Anat. d. Haustiere.)
]

The =lateral borders= are rough, thick in front, thin behind.

The =base= (Basis ossis sacri) is directed forward, and is relatively very wide. It presents centrally the =body= of the first sacral segment, which is wide transversely, flattened dorso-ventrally, and has a rounded surface which articulates with the last lumbar vertebra through the medium of an intervertebral fibro-cartilage. The ventral margin projects slightly, forming the =promontory= (Promontorium). Above the body is the entrance to the sacral canal, flanked by a pair of =articular processes=, which project upward and forward from the arch, and have concave surfaces internally for articulation with those of the last lumbar vertebra. On each side of these is a smooth notch which is converted into a foramen by apposition with the last lumbar. The lateral parts of the base, the =wings= or =alæ= (Alæ sacrales), are strong prismatic masses with pointed ends, which result from the fusion of the first with part of the second transverse process. Each has in front a large, oval, slightly convex surface for articulation with the transverse process of the last lumbar. Posteriorly there is an elongated oval area which faces upward, backward, and outward. This is the =auricular surface= (Facies auricularis), which articulates with the ilium; it is slightly concave in its length, and somewhat rough and irregular. The rest of the dorsal surface of the wing is roughened for ligamentous attachment, while the ventral surface is smooth.

The =apex= (Apex ossis sacri) is the posterior aspect of the last sacral vertebra and is quite small. It presents the elliptical flattened surface of the body, above which is the triangular posterior opening of the sacral canal, surmounted by the last sacral spine. There is a pair of narrow notches between the arch and body, above which rudiments of articular processes may occur.

The name =sacral canal= (Canalis sacralis) is applied to that part of the vertebral canal which traverses the sacrum. Its anterior part is large and has the form of a triangle with the angles rounded off; its width is about twice its height. Traced backward it is seen to diminish in size rapidly, and the posterior opening is quite small and triangular.

The term =lateral part= (Pars lateralis) designates the portion external to the foramina, which results from the fusion of the transverse processes.

FIG. 20.—FIRST COCCYGEAL VERTEBRA OF HORSE, LEFT VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 21.—FIRST COCCYGEAL VERTEBRA OF HORSE, DORSAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

=Development.=—The several sacral vertebræ ossify in the typical manner. Separate centers for costal elements in the lateral parts have not yet been found in the domesticated animals. Fusion begins in front, and is usually not complete behind till near adult age. The lateral parts unite before the bodies. It is rather curious that the epiphyseal plates of adjacent segments unite with each other before they fuse with the main portion of the bodies.

THE COCCYGEAL VERTEBRÆ

These (Vertebræ coccygeæ) vary considerably in number, but eighteen may be taken as an average. From first to last they become reduced in size and, with the exception of a few at the beginning of the series, consist of bodies only. The first three have bodies which are somewhat flattened dorso-ventrally, constricted in the middle, and have at the ends slightly convex, elliptical, articular surfaces. The ventral surface has a median groove for the coccygeal artery. The arch is small and triangular; it is formed of two flat plates which are prolonged to form a short spinous process with a thickened and often double summit. The anterior notches are absent. Functional articular processes are not present, but small rudiments of the anterior pair commonly occur. The transverse processes are relatively large plates which project horizontally outward. Further back the arch becomes incomplete, open above, and soon disappears; the transverse processes gradually fade out, and the vertebræ are reduced to cylindrical rods of diminishing size. The last one has a pointed end.

=Variations.=—The number is said by good observers to vary between
fourteen and twenty-one. In old age the first is often fused with the
sacrum, and sometimes with the second. The arch of the third may be
open.

THE VERTEBRAL COLUMN AS A WHOLE

In the mid-dorsal line is the series of =spinous processes=, which are low ridges in the cervical region with the exception of the second and seventh, reach their maximum height at the fourth and fifth thoracic vertebræ, and diminish to the fifteenth or sixteenth thoracic. Behind this they are about equal in height as far as the last lumbar and first sacral, which are somewhat lower. The second sacral spine is about as high as the middle lumbar; behind this they diminish rather rapidly in height and fade out about the third coccygeal. Their =inclination= backward is most decided at the second thoracic, diminishes from the sixth or seventh to the fifteenth or sixteenth, which is vertical and is termed the =anticlinal= or =diaphragmatic vertebra.= Behind this they are inclined a little forward until the sacrum is reached; here there is an abrupt change to the backward inclination, so that a considerable interspinous angle is formed.

On either side of the spinous processes is a =vertebral groove= which contains the deep muscles of the spine. The floor of the groove is formed by the laminæ and articular processes. It is wide in the neck and narrows progressively in the back.

Viewed from the side, the column presents a series of curves. When the head and neck are in the ordinary neutral position, the anterior part of the cervical spine forms a gentle curve, concave ventrally. The posterior cervical and first thoracic vertebræ form a more pronounced curve in the opposite direction. At the junction of the cervical and thoracic regions there is a marked change of direction, forming a ventral projection or angle. At the second thoracic vertebra a gentle curve, concave ventrally, begins. This is continued to the lumbo-sacral junction, where there is a change of direction, and hence a promontory. The sacrum has a variable, but never very pronounced, ventral concave curvature, which is continued in a much accentuated form in the coccygeal region. It may be remarked that a line through the summits of the spines does not correspond to these curves formed by the bodies.

The =vertebral canal=, of course, corresponds in curvature to the bodies. Its =caliber= varies greatly at different points. The greatest diameter (ca. 5 cm.) is in the atlas, where it contains the dens of the axis in addition to the spinal cord, and provision must be made for extensive movement. It is very much smaller in the axis (ca. 2.5 cm. wide, 3 cm. high). It widens considerably at the junction of the cervical and thoracic regions to accommodate the cervical enlargement of the spinal cord. Beyond this it diminishes, and is smaller in the middle of the back than at any preceding point; this is correlated with the small size of the spinal cord and the very limited movement of the spine. At the middle of the lumbar region it again widens considerably to contain the lumbar enlargement of the spinal cord. The caliber diminishes very rapidly from the second sacral segment backward, and the canal ceases to be complete at the fourth coccygeal vertebra.

The =articular processes= are very large and wide apart in the neck, greatly reduced and much closer together in the back, larger and interlocking in the lumbar region.

The =transverse processes= are large and outstanding in the neck, where they form the outer boundary of a ventral groove occupied by the longus colli muscle. In the back they are short and stout, and are characterized by the facets for the tubercles of the ribs. On the first thoracic vertebra this facet is large, deeply concave, and situated almost directly outward from the cavity for the head of the rib; traced backward it becomes smaller and flatter, and gradually comes to lie behind the cavity for the head of the rib, with which it is fused on the last and often also on the next to the last thoracic vertebra. The processes in the lumbar region have a characteristic elongated plate-like form. In the sacral region they are fused to form the wings and lateral parts of the sacrum. In the coccygeal region they are at first of considerable size relatively, but undergo rapid reduction, and disappear at the fifth or sixth vertebra.

The cavities for the heads of the ribs diminish progressively in size and depth from first to last.

The =mammillary processes= are usually distinct on the fourteenth to the seventeenth thoracic vertebræ. In front of these they blend with the transverse, behind with the anterior articular processes.

The length of the vertebral column (including the intervertebral
fibro-cartilages) in a horse of medium size is about 260 to 265 cm.
(ca. 8 feet 8 inches to 8 feet 10 inches). The relative lengths of the
various regions appear to vary most in the neck and back.

The table below gives the measurements in centimeters in a trotting
stallion of medium size and in an adult Percheron stallion. The
percentages are in round numbers.

─────────┬───────────┬───────────
│ TROTTER │ PERCHERON
─────────┼─────┬─────┼─────┬─────
Cervical │ 60.0│22.7%│ 74.0│25.4%
Thoracic │ 98.0│37.2%│100.0│34.4%
Lumbar │ 31.5│12.0%│ 36.5│12.5%
Sacral │ 20.0│ 7.6%│ 22.5│ 7.7%
Coccygeal│ 54.0│20.5%│ 58.0│20.0%
─────────┼─────┼─────┼─────┼─────
│263.5│100.0│291.0│100.0
─────────┴─────┴─────┴─────┴─────

THE RIBS

There are usually eighteen pairs of ribs in the horse, but a nineteenth rib on one side or both is not at all rare. Eight are =sternal= or “true” ribs, the remainder =asternal= or “false.” Ribs from different parts of the series vary much in length, curvature, and other characters. We will therefore consider as a type a rib from the middle of the series first, and afterward note the chief serial differences. A =typical rib= has the following characters:

The =shaft= or =body= (Corpus costæ) is elongated, relatively very narrow, and strongly curved; the curvature is most pronounced in the dorsal third, and the ventral part is twisted and inclined inward, so that when a rib is laid with its outer surface on the table, the ventral end is raised. The =external surface= is convex in its length and also transversely; its anterior part is, however, grooved longitudinally. A distinct angle, _i. e._, a point at which the curve of the rib changes rather suddenly, as in man, can scarcely be said to exist in the horse. The term is often applied, however, to a corresponding rough elevation which gives attachment to the ilio-costalis muscle; it is most distinct on the fourth to the eighth inclusive. The =internal surface= is smooth, concave in its length, and rounded from side to side; the costal groove, situated posteriorly, is very distinct above and fades out about the middle. The =anterior border= is concave, the =posterior= convex.

The =vertebral extremity= (Extremitas vertebralis) consists of the head, neck, and tubercle. The =head= (Capitulum costæ) has an articular surface (Facies articularis capituli costæ), composed of two convex facets, anterior and posterior, separated by a groove for the attachment of the conjugal ligament. It articulates with the cavity formed by facets on the bodies of two adjacent thoracic vertebræ and the intervertebral fibro-cartilage. The _neck_ (Collum costæ) is roughened above and in front. The =tubercle= (Tuberculum costæ) is placed above and behind the junction of neck and shaft; it has a small surface (Facies articularis tuberculi costæ) for articulation with the transverse process of the corresponding thoracic vertebra.

The =sternal extremity= (Extremitas sternalis) is somewhat expanded, and is continued by the costal cartilage.

The =first rib= is easily distinguished. It is the shortest and least curved. At the lower part of the anterior border there is a smooth impression where the brachial vein curves around it; above this is usually the scalene tubercle. The costal groove is absent. The head is large and has two facets of unequal extent which meet at an acute angle in front; the smaller one faces forward and articulates with the last cervical vertebra; the larger one is directed inward and articulates with the first thoracic vertebra. The neck is thick and very short. The tubercle is larger than that of any other rib and has an extensive articular surface which is convex in its length. The sternal end is larger than that of any other rib; it is thick and very wide, and is turned a little forward.

FIG. 22.—LEFT SEVENTH RIB OF HORSE, ANTERO-EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =last rib= is the most slender and regularly curved. It is usually but little longer than the second. The facet on the tubercle is confluent with that of the head. (This feature, however, is usual on the seventeenth also, and not rare on the sixteenth.)

The serial position of the other ribs may be determined approximately by the following considerations: The =length= increases from the first to the tenth and eleventh and then diminishes. The =width= increases somewhat to the sixth and then diminishes. The =anterior border= is thin and sharp from the second to the eighth, and behind this becomes thick and rounded. The =external groove= is distinct on the fourth to the eighth inclusive. The =curvature= increases in degree rapidly from the first to the seventh, remains about the same to the sixteenth, and then decreases very noticeably. In regard to dorso-ventral =direction=, the first rib inclines a little forward, the second is about vertical, while behind this they slope backward in increasing degree, so that a plane tangent to the ventral ends of the last pair cuts the third lumbar vertebra. The =head= and =tubercle= diminish in size from first to last. Their relative positions change in that the tubercle of the first rib lies almost directly outward from the head, while further back it gradually comes to lie behind it. The =neck= is longest on the longest ribs, and is absent on the last two or three. A costo-transverse foramen is formed between the neck and the transverse process.

=Development.=—The ribs ossify in cartilage from =three centers=, one each for the shaft, head, and tubercle; the third center is absent in some of the posterior ribs.

=Variations.=—A nineteenth rib on one side or both is not rare. It is
usually imperfectly developed and quite variable. In some cases it is
a mere strip of cartilage connected by ligament with the first lumbar
transverse process; in other cases it is ossified, and may be fused
with the process; in others again it is connected with an additional
presacral vertebra which may be thoracic or lumbar or ambiguous in
character. It is commonly floating, but may be attached to the
eighteenth. Reduction in number is uncommon. Fusion of adjacent ribs
sometimes occurs.

THE COSTAL CARTILAGES

FIG. 23.—STERNUM AND COSTAL CARTILAGES OF HORSE, VENTRAL VIEW. (After
Ellenberger-Baum, Anat. f. Künstler.)
]

The first costal cartilage is an inch or more (2.5 to 3 cm.) in length. The upper part is very wide and thick. The sternal end is small. The two articulate with each other as well as with the sternum. The cartilages of the other sternal ribs increase progressively in length and become more rounded. The sternal end is expanded and has an elliptical convex facet for articulation with the sternum. The cartilages of the asternal ribs are long, slender, and pointed. The ninth is the longest, and is very firmly attached to the eighth; behind this they diminish progressively in size, and are attached to each other by elastic tissue. Except in the case of the first, the cartilage does not continue the direction of the rib, but forms with the latter an angle which is open in front, and decreases in obtuseness from second to last.

More or less extensive ossification is to be regarded as a normal occurrence, especially in the cartilages of the sternal ribs.

THE STERNUM

The sternum of the horse is shaped somewhat like a canoe; it is compressed laterally, except in its posterior part, which is flattened dorso-ventrally. It is inclined obliquely so that the posterior end is about six to eight inches (15 to 20 cm.) lower than the anterior.

The =dorsal surface= has the form of a very narrow isosceles triangle with the apex in front. It is concave longitudinally, flattened transversely.

The =lateral surfaces= are convex above, slightly concave below, and diminish in extent behind. Each presents on its upper part seven =articular cavities= (Foveæ costales), which receive the sternal ends of the second to the eighth costal cartilages inclusive. These cavities are situated in series at the intersternebral junctions. The first four are elliptical in outline with the long diameter vertical, and are separated by considerable regular intervals. The others are progressively smaller, more circular, and closer together. The area below these cavities gives attachment to the pectoral muscles.

FIG. 24.—STERNUM OF HORSE, LATERAL VIEW.

The sternebræ are designated by Roman numerals and the costal facets
by ordinary figures.
]

The =dorso-lateral= borders separate the dorsal and lateral surfaces. They give attachment to the lateral branches of the sternal ligament.

The =ventral border= forms the prominent keel-like crest of the sternum (Crista sterni) which may be felt in the living animal; it fades out behind.

FIG. 25.—ANTERIOR APERTURE OF THORAX OF HORSE. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]

The =anterior extremity= or =manubrium sterni=[5] can be distinctly felt in the central furrow of the breast. It consists of a laterally compressed cartilaginous prolongation, commonly called the =cariniform cartilage=. Its lateral surfaces are flat and furnish attachment to muscles of the breast and neck. The ventral border is rounded, and is continued backward on the body of the bone. The dorsal border is concave and has an articular cavity for the first pair of costal cartilages.

The =posterior extremity= is formed by the =xiphoid= or =ensiform cartilage= (Processus xiphoideus). This is a thin plate, connected in front with the last bony segment by a relatively thick, narrow neck, and expanding in nearly circular form behind and laterally. Its dorsal surface is concave, and gives attachment to the diaphragm. The ventral surface is convex. The free margin is very thin.

=Development.=—At birth the sternum of the horse consists of seven bony segments or sternebræ united by intersternebral cartilages. The last two sternebræ fuse in the second month, but the others do not usually unite completely even in old age. The sternebræ consist of very vascular spongy bone covered by a very thin layer of compact substance. The adult sternum thus consists to a very considerable extent of persisting cartilage, viz., the intersternebral cartilages, the ventral keel, and the extremities; in old age these undergo partial ossification.

THE THORAX

The bony thorax of the horse is remarkably compressed laterally in its anterior part, but widens greatly behind. The =anterior aperture= is oval and very narrow below; in a horse of medium size its greatest width is about 4 inches (10 cm.), and its height 7 to 8 inches (ca. 18 to 20 cm.). The =ventral wall= or =floor= is about 10 inches (40 cm.) long, and the =dorsal wall= or =roof= about 38 to 40 inches (95 to 100 cm.) long. The height from the last segment of the sternum to the seventh or eighth thoracic vertebra is about twice that of the anterior aperture; this is due to the obliquity and divergence of the roof and floor. The greatest width of the posterior aperture is about 20 to 24 inches (50 to 60 cm.). The intercostal spaces increase in width from the first to the seventh or eighth, and then diminish. Their average width is about 1¼ to 1½ inches (3 to 3.5 cm.).

THE BONES OF THE SKULL

(A) BONES OF THE CRANIUM

The bones of the cranium (Ossa cranii) are the Occipital, Sphenoid, Ethmoid, Interparietal, Parietal, Frontal, and Temporal. The first four are single, the others paired.

FIG. 26.—SKULL, ATLAS, AND AXIS OF HORSE, LATERAL VIEW.

8, Body of mandible; 28′, horizontal (molar) part of ramus; 30,
vertical part of ramus; 9, zygomatic process of squamous temporal;
11, coronoid process of mandible; 12, supraorbital process; 13″,
paramastoid (styloid) process of occipital; 19, orbit; 20, malar
bone; 21, lacrimal bone; 22, nasal bone; 23, premaxilla; 23′, nasal
process of premaxilla; 25, 29, canine teeth; 26, maxilla; 27, facial
crest; 31, condyle of mandible; 32, atlas; 33, axis; x, wing of
atlas; e, naso-maxillary notch. (After Ellenberger-Baum, Anat. für
Künstler.)
]

THE OCCIPITAL BONE

The =occipital bone= (Os occipitale) is situated at the posterior part of the cranium, of which it forms the posterior wall and part of the ventral wall or base.[6]

Its lower part is perforated centrally by a large, almost circular opening, the =foramen magnum= (Foramen occipitale magnum), at which the brain and spinal cord join. The foramen is bounded laterally and dorsally by the =lateral parts= of the bone, and ventrally by the =basilar part= or =process=. Above the lateral parts—but not entering into the formation of the foramen magnum—is the =squamous part=.

FIG. 27.—LINE DRAWING OF POSTERIOR HALF OF BASE OF SKULL OF HORSE,
WITHOUT MANDIBLE. (Key to Fig. 28.)

1, Incisura carotica; 2, incisura ovalis; 3, incisura spinosa; 4,
external orifice of parieto-temporal canal; 5, Eustachian canal; 6,
petro-tympanic fissure; 7, external auditory canal; 8, hyoid
process; 9, Vidian groove; 10, supraorbital process; _A_, basilar
part of occipital; _B_, body of sphenoid; _C_, temporal wing of
sphenoid; _D_, squamous temporal bone; _E_, petrous temporal bone;
_F_, orbital part of frontal bone.
]

The =lateral parts= (Partes laterales) bear the =occipital condyles= (Condyli occipitales), which articulate with the atlas. The condyles are obliquely placed, wide apart dorsally, and separated by a small interval ventrally. The articular surface is curved so sharply in the dorso-ventral direction as to form a blunt ridge externally. The cranial surface is concave and smooth. External to the condyle is the =paramastoid= or =styloid process= (Processus jugularis), a strong flattened bar of bone which projects downward and backward; its external surface is convex and roughened for muscular attachment. Between the root of this process and the condyle is a smooth depression, the =condyloid fossa= (Fossa condyloidea inferior); in this is the =hypoglossal foramen= (Foramen hypoglossi), which transmits the nerve of like name.

The =basilar part= or =process= (Pars basilaris) is a strong, somewhat prismatic bar which extends forward from the ventral margin of the foramen magnum. It is much narrower in front than behind. The ventral surface is rounded. The cranial surface is concave and smooth; its posterior part, supports the medulla, and its anterior part has a shallow cavity on which the pons rests. The lateral borders are thin and sharp, and form the inner margin of the =foramen lacerum= (Foramen lacerum et jugulare). The anterior end has, in the young subject, a semicircular, flat, pitted surface which is attached to the body of the sphenoid bone by a layer of cartilage; in the adult there is complete fusion. On the ventral aspect of the junction are tubercles for the attachment of the ventral straight muscles of the head.

FIG. 28.—VENTRAL SURFACE OF SKULL OF HORSE, POSTERIOR HALF WITHOUT
MANDIBLE.

The skull is inclined slightly. (Notation on key Fig. 27.)
]

The =squamous part= (Squama occipitalis) is the somewhat quadrilateral mass situated above the lateral portions, from which it remains distinct till the second year. The =outer surface= is crossed by a very prominent ridge, the =occipital crest=; the middle part of this is thick, transverse in direction, and forms the highest point of the skull when the head is in the ordinary position; laterally it becomes thinner and runs downward and forward to join the temporal crest.[7] The crest divides the surface into two very unequal parts; the small anterior area (Planum parietale) presents a median ridge which is the posterior part of the external sagittal crest; the large area below the crest (Planum nuchale) also has a central eminence, the =external occipital protuberance=, on the sides of which the funicular part of the ligamentum nuchæ is attached. The =internal surface= is concave and presents a deep central depression and two shallower lateral ones which adapt it to the surface of the cerebellum.

The occipital bone is connected by suture with the interparietal, two parietals, and two temporals, and by synchondrosis with the sphenoid; the condyles articulate with the atlas.

=Development.=—The occipital ossifies in cartilage from =four centers=, and consists at birth of four pieces as described above.[8] The lateral parts unite with the basilar at three to four months, and with the squama in the second year, when the bone is consolidated.

The parieto-occipital suture and the spheno-occipital synchondrosis are obliterated about the fifth year usually. The temporo-occipital suture partially ossifies in old subjects.

THE SPHENOID BONE

The =sphenoid bone= (Os sphenoidale) is situated in the base of the cranium, its central part lying in front of the basilar part of the occipital. It consists of a central part, the body, two pairs of wings, and two pterygoid processes.

The =body= (Corpus) is a cylindrical bar, flattened dorso-ventrally, and wider in front than behind. Its =ventral surface= (Facies externa) is convex in the transverse direction; and its anterior part is concealed to a large extent by the vomer and pterygoid bones. The =cerebral surface= (Facies cerebralis) presents the following features: (1) In front is a raised, flattened part which is subdivided by a median elevation into two slightly concave lateral areas; this part has a posterior, thin, free margin (Limbus sphenoidalis), which overlies the entrance to the optic foramina. (2) Just behind this and at a lower level is a smooth transverse depression, the =optic groove= (Sulcus chiasmatis), on which the optic chiasma rests. (3) From each end of this groove the =optic foramen= passes forward and outward to terminate in the posterior part of the orbital fossa.[9] (4) Near the posterior end is a central depression, the =hypophyseal= or =pituitary fossa= (Fossa hypophysea), which lodges the hypophysis cerebri or pituitary body. On each side of this is an ill-defined groove for the internal carotid artery and the cavernous sinus. The =anterior end= is expanded, and is excavated to form the =sphenoidal sinuses.= These cavities extend back as far as the optic groove, and are usually continuous in front with the cavities in the vertical parts of the palate bones; they are separated by a complete septum which is not always median.[10] The =posterior end= is flat and is joined to the basilar part of the occipital; at the line of junction there is dorsally a transverse elevation, the =spheno-occipital= crest (Crista sphenooccipitalis).

The =orbital wings= (Alæ orbitales) curve upward and somewhat outward from the sides of the body of the presphenoid. Their =inner= or =cerebral surfaces= are concave, and are marked by =digital impressions= (Impressiones digitatæ) for the gyri of the cerebrum. The =external surface= is convex and is largely concealed by the overlapping temporal wing and the squamous temporal and frontal bones; a narrow part of it (Facies orbitalis) is uncovered on the inner wail of the orbital cavity at the sphenoidal notch of the frontal bone. The =dorsal border= unites with the frontal bone at the spheno-frontal suture. The =anterior border= joins the ethmoid at the spheno-ethmoidal suture; at its lower part it concurs with the frontal in the formation of the =ethmoidal= (or internal orbital) =foramen=. The =posterior border= is overlapped by the temporal wing and the squamous temporal. The =root= of the wing is perforated by the =optic foramen=. Immediately below and behind the latter (_i. e._, beneath the root) is the =foramen lacerum orbitale= or =orbital fissure=. Below this, and separated from it usually by a thin plate, is a larger opening, the =foramen rotundum=, which is bounded externally by the root of the pterygoid process. Behind these foramina is the =pterygoid crest= (Crista pterygoidea), which is continued downward and forward on the pterygoid process; on its upper part may be found the small and inconstant =trochlear= (or pathetic) =foramen=. Just behind the crest is the =temporal foramen= (For. alare parvum), through which the anterior deep temporal artery emerges from the alar canal of the pterygoid process.

The =temporal wings= (Alæ temporales) extend outward and somewhat upward from the body of the postsphenoid; they are irregularly quadrilateral in outline. The =external surface= (Facies temporalis) enters into the formation of the infratemporal fossa, and bears the pterygoid process on its anterior part; at the junction with the body there is a small groove for the pterygoid nerve. The =internal surface= (Facies cerebralis) presents, at the junction with the body, two longitudinal =grooves= (Sulci nervorum). The outer groove is the larger, and leads forward to the foramen rotundum; it contains the maxillary nerve. The inner groove conducts to the orbital fissure, and contains the third, sixth, and ophthalmic nerves. The outer groove is bounded externally by a thin overhanging crest, on which is a small groove for the fourth nerve. The remainder of the surface is concave and supports the pyriform lobe of the brain. The =dorsal border= joins the squamous temporal at the spheno-squamous suture. The =anterior border= joins the orbital wing. The =posterior border= forms the anterior boundary of the foramen lacerum; it presents =three notches=, which are (from within outward) the carotid, oval, and spinous (Incisura carotica, ovalis, spinosa). The angle of junction of the dorsal and posterior borders articulates with the parietal bone.

The =pterygoid processes= (Processus pterygoidei) arise from the temporal wings and the body. They project downward and forward, and curve outward at the lower part. The root is perforated by the =alar canal= (Canalis alaris),[11] which transmits the internal maxillary artery. From this canal a branch leads upward and forward to open at the temporal foramen. The =external surface= is concave, and is marked by lines for muscular attachment. The =internal surface= is largely concealed by the overlapping palate and pterygoid bones, with which it concurs in the formation of the =pterygoid= or =Vidian canal=.

=Development.=—The sphenoid is ossified in cartilage, and consists in early life of two distinct parts, the presphenoid and postsphenoid. The former develops from two centers, one in each wing; the latter has three centers, one for the body and one for each wing. The pterygoid processes ossify from the centers of the temporal wings.

=Variation.=—The dorsal border of the orbital wing may come to the
surface through a defect in the frontal bone at the place where the
horn process is situated in animals which have frontal horns.

THE ETHMOID BONE

The =ethmoid= (Os ethmoidale) is situated in front of the body and orbital wings of the sphenoid. It projects forward between the orbital plates of the frontal bones and enters into the formation of the cranial, nasal, and paranasal cavities.[12] It consists of four parts—the cribriform plate, two lateral masses, and the perpendicular plate.

The =cribriform plate= (Lamina cribrosa) is a sieve-like partition between the cranial and nasal cavities. Its margin joins the orbital wings of the sphenoid laterally, and the cranial plate of the frontal bones dorsally. Its =cranial surface= is divided into two parts by a median ridge, the =crista galli=, which is the intracranial portion of the perpendicular plate. Each half forms a deep oval cavity, the =ethmoidal= or =olfactory fossa=, which lodges the olfactory bulb. The plate is perforated by numerous small foramina for the passage of the olfactory nerve filaments, and on either side is the much larger =ethmoidal foramen.= The =nasal surface= is convex, and has the lateral masses attached to it.

The =lateral masses= or =labyrinth= project forward from the cribriform plate into the posterior part of the nasal cavity, which they nearly fill. Each mass is somewhat conical in shape, with the base attached to half of the cribriform plate. The =inner surface= is separated by a narrow space from the perpendicular plate. The =outer surface= is convex and faces chiefly into the frontal and maxillary sinuses, but is attached behind to the inner wall of the orbital cavity; it is covered by a very thin layer of bone, the =lamina papyracea.= The mass consists of a large number of delicate, scroll-like plates of bone, termed =ethmoturbinals= or =ethmoidal cells.= These are attached to the lamina papyracea, and are separated by narrow intervals termed =ethmoidal meatuses=, which communicate with the nasal cavity. In the living animal the ethmoturbinals are covered with mucous membrane.

FIG. 29.—CROSS-SECTION OF LATERAL MASS OF ETHMOID BONE OF HORSE.
]

The lateral mass is a very complex structure, the arrangement of which
may be studied on cross-sections of decalcified specimens with the
mucous membrane retained. Each mass consists of six turbinals which
extend almost to the perpendicular plate and are termed
=endoturbinals=. These diminish in size from above downward; the
largest is attached to the nasal bone, and is hence usually called the
naso-turbinal or superior turbinal; the second is much smaller, and is
very commonly termed the great ethmoid cell. Between the endoturbinals
are twenty-one small =ecto=turbinals, and all are beset with secondary
and tertiary coiled lamellæ.

The =perpendicular plate= or =mesethmoid= (Lamina perpendicularis) is median, and forms the posterior part of the septum nasi. Its =lateral surfaces= are nearly plane, but are marked below by some grooves and ridges; they are covered by the nasal mucous membrane. The =anterior border= is irregular and is continuous with the septal cartilage. The =posterior border= projects into the cranial cavity as a ridge, the =crista galli.= The =dorsal border= joins the frontal bones at their line of junction—the frontal suture. The =ventral border= is received into the groove of the vomer.

=Development.=—The ethmoid develops in cartilage from five centers, two for each lateral mass, and one for the perpendicular plate; from the latter ossification extends into the cribriform plate. At birth the perpendicular and cribriform plates are entirely cartilaginous. By the time ossification is complete the ethmoid has united with surrounding bones to such an extent that it cannot be separated intact for study.

FIG. 30.—SKULL OF HORSE, DORSAL VIEW.

12, Supraorbital process; 14, parietal bone; 14′, external sagittal
crest; 15, frontal bone; 15′, frontal crest; 21, lacrimal bone; 22,
nasal bone; 26, maxilla; 24, incisor teeth. (After Ellenberger-Baum,
Anat. für Künstler.)
]

THE INTERPARIETAL BONE

This bone (Os interparietale) is centrally placed between the squamous part of the occipital and the parietal bones. It is usually described as a single bone, although it ossifies from two chief lateral centers, and is sometimes distinctly paired in skulls of young foals.

The =external surface= (Facies externa) is quadrilateral and is flat and smooth in the very young foal; later it presents the median external sagittal crest.

The =internal surface= (Facies cerebralis) presents the =internal occipital protuberance=, a three-sided process which projects downward and forward into the cranial cavity between the cerebral hemispheres and the cerebellum; it has three concave surfaces and three sharp borders which form part of the tentorium osseum.

The =posterior border= is thick; it joins the squamous part of the occipital bone. The =lateral= and =anterior borders= are united by suture with the parietal bones.

=Development.=—The interparietal ossifies in membrane from two chief lateral centers.[13] It fuses first with the parietals, somewhat later with the occipital, but the period at which this union takes place is quite variable.

THE PARIETAL BONES

The two =parietal bones= (Ossa parietalia) form the greater part of the roof of the cranium; they unite in the median line, forming the =sagittal suture=. Each is quadrilateral in outline and has two surfaces and four borders.

The =external surface= (Facies parietalis) is convex, and is marked by a more or less prominent curved line, the =external sagittal crest=; this is median in its posterior part, and is continuous with the crest of like name on the occipital bone; in front it curves outward and is continuous with the frontal crest. The surface external to the crest (Planum temporale) enters into the formation of the temporal fossa, and is roughened for the attachment of the temporal muscle.

The =internal= or =cerebral surface= (Facies cerebralis) is concave. It presents numerous =digital impressions= (Impressiones digitatæ) which correspond to the gyri of the cerebrum. There are also furrows (Sulci vasculosi) for the meningeal arteries. Along the inner border there is a =sagittal groove= (Sulcus sagittalis) for the superior longitudinal sinus.

The =anterior border= joins the frontal bone at the =parieto-frontal suture= (Sutura coronalis).

The =posterior border= meets the occipital bone at the =parieto-occipital suture= (Sutura lambdoidea). Below this junction it curves inward and concurs with the temporal bone in the formation of the =parieto-temporal canal= (Meatus temporalis). A =transverse groove= (Sulcus transversus) connects this canal with the sagittal sulcus.

The =internal border= is thick and serrated. It joins its fellow at the =sagittal suture=, and (in the young subject) meets the interparietal at the =interparietal suture=. The line of junction is marked internally by the =internal sagittal crest= (Crista sagittalis interna).

The =external border= is beveled and is overlapped by the squamous temporal bone, forming the parieto-temporal suture (Sutura parieto-squamosa). The angle of junction of the external and posterior borders articulates with the posterior angle of the temporal wing of the sphenoid.

=Development.=—Each parietal bone ossifies in membrane from a single center. In the young foal the central part of the bone is much more convex than in the adult and forms a prominence similar to the pronounced tuber parietale of the young child; the external sagittal crest is not present, and the external surface is smooth.

The sagittal suture is usually closed at four years, the
parieto-occipital at five years, and the parieto-temporal at twelve to
fifteen years.

THE FRONTAL BONES

The =frontal bones= (Ossa frontalia) are situated on the limits of the cranium and face, between the parietals behind and the nasal bones in front. Each is irregularly quadrilateral, and consists of frontal, orbital, and temporal parts.

The =frontal part= (Pars naso-frontalis) forms the basis of the forehead. Its =external surface= (Facies frontalis) is nearly flat, and is smooth and subcutaneous; it is separated from the temporal part by the external frontal crest (Crista frontalis externa). At the junction with the orbital part the =supraorbital= or =zygomatic process= (Proc. zygomaticus) curves outward and downward to join the zygomatic arch. The process partially separates the orbit from the temporal fossa; its root is perforated by the =supraorbital foramen=, or presents instead a notch on its anterior border; its upper surface is convex, while the lower or orbital surface is concave and smooth, forming a shallow fossa for the lacrimal gland (Fossa glandulæ lacrimalis). The =internal surface= enters into the formation of the cranial cavity and the frontal sinus. The cranial surface presents digital impressions for the cerebral gyri. The two plates of the bone separate and diverge in front, and thus inclose a large air-space which is part of the =frontal sinus=. The cranial plate curves downward and articulates with the cribriform plate of the ethmoid bone; the facial plate extends forward and joins the nasal and lacrimal bones.

The =orbital part= (Pars orbitalis) forms the major part of the inner wall of the orbital cavity. It is separated from the frontal part by a prominent ridge which is part of the orbital margin. Its =external= or =orbital surface= is concave and smooth, and presents superiorly a small depression (Fovea trochlearis), which is bridged by a small bar of cartilage, around which the superior oblique muscle of the eye is reflected. The lower border concurs with the orbital wing of the sphenoid in the formation of the ethmoidal or internal orbital foramen. The =internal surface= faces into the frontal sinus and gives attachment to the lateral mass of the ethmoid.

The =temporal part= is separated from the orbital part by the deep =sphenoidal notch= (Incisura sphenoidalis), which is occupied by the orbital wing of the sphenoid. Its =external surface= forms part of the inner wall of the temporal fossa. The =internal surface= is largely covered by the orbital wing of the sphenoid in the young subject, but later forms part of the wall of the frontal sinus.

The principal connections of the frontal bone are as follows: (1) The
inner border joins its fellow at the =frontal suture=. (2) The
anterior border meets the nasal and lacrimal at the =naso-frontal= and
=fronto-lacrimal sutures=. (3) Laterally it forms the =spheno-frontal=
suture with the orbital wing of the sphenoid, and also joins the
palate bone and maxilla. (4) Posteriorly it meets the parietal at the
=parieto-frontal= (or coronal) =suture=, and articulates below this
with the squamous temporal. (5) The extremity of the supraorbital
process unites with the zygomatic process of the temporal bone.

=Development.=—Each ossifies in membrane from one center which appears in the root of the supraorbital process. In the new-born foal there is a slit between the cranial plate and the orbital and temporal plates which receives the cartilaginous margin of the orbital wing of the sphenoid.

THE TEMPORAL BONES

The =temporal bone= (Os temporale) forms the greater part of the lateral wall of the cranium. It is situated between the occipital behind, the parietal above, the frontal in front, and the sphenoid below. It consists of two distinct parts, =squamous= and =petrous=.

1. The =squamous temporal= (Squama temporalis) is a shell-like plate which has two surfaces and four borders.

The =internal surface= (Facies cerebralis) is largely overlapped by the surrounding bones, but its central part is free and presents digital impressions and vascular grooves.

The =external surface= (Facies temporalis) is convex, and enters into the formation of the temporal fossa. From its lower part there springs the =zygomatic process= (Processus zygomaticus), which forms the external boundary of the temporal fossa. It is at first directed outward, and is wide and flattened dorso-ventrally. It then turns forward, becomes narrower, and is twisted so that its surfaces are internal and external. Its anterior end is pointed and joins the zygomatic process of the malar bone, with which it forms the =zygomatic arch= (Arcus zygomaticus). The narrow anterior part has a convex outer surface and a concave inner one. Its upper border has a rough area for articulation with the supraorbital process of the frontal. Its lower border is wide and rough. The wide posterior part presents on its ventral face a surface for articulation with the condyle of the mandible. This surface consists of a transversely elongated =condyle= (Tuberculum articulare), behind which is the =glenoid fossa= (Fossa mandibularis). The fossa is limited behind by the =postglenoid process=, the anterior surface of which is articular. Behind this process is a fossa in which is the external opening of the parieto-temporal canal. The dorsal surface is concave and forms the outer boundary of the temporal fossa. The superior border is sinuous and is continuous behind with the temporal crest.

The =posterior process= (Processus posterior) springs from the posterior part of the squama. Its external surface is crossed by the temporal crest, which forms here the outer limit of the temporal fossa. The internal surface forms the outer boundary of the parieto-temporal canal, and is elsewhere applied to the petrous portion. It divides into two branches, upper and lower; the upper branch unites with the occipital bone, while the lower one curves downward behind the external auditory process and overlaps the mastoid process.

The =superior border= of the squamous temporal articulates with the parietal, forming the =parieto-temporal suture=. The =inferior border= joins the temporal wing of the sphenoid at the =spheno-squamous suture=. The =anterior border= unites with the frontal bone, and the =posterior= with the parietal.

2. The =petrous temporal= (Os petrosum) is placed between the occipital behind and the parietal in front, and is largely overlapped externally by the squamous temporal. It has the form of a four-sided pyramid, the base of which is ventral.

The =external surface= is mainly concealed by the squamous temporal, but two features are visible. A short tube of bone, the =external auditory process=, protrudes from the lowest part through the notch of the squamous temporal. The process is directed outward, upward, and a little forward. It gives attachment to the annular cartilage of the ear. Its lumen, the =external auditory meatus= (Meatus acusticus externus), conducts to the cavity of the middle ear (tympanum) in the dry skull, but is separated from it by the tympanic membrane in the natural state. The =mastoid process= projects ventrally in the interval between the posterior process of the squamous temporal and the root of the paramastoid (or styloid) process of the occipital bone; its outer surface is crossed by a curved groove which leads to the parieto-temporal canal.

The =internal surface= faces into the cerebellar fossa of the cranium. It is concave and smooth but irregular. In its lower part is the entrance to a short canal, the =internal auditory meatus=, which transmits the seventh and eighth cranial nerves.