Chapter V: Muscles of the Leg and Foot (6)

The liver is relatively large, its average weight being about 4 to 4½ pounds (ca. 2 kg.). It is thick and very strongly curved.[114] It is divided by three deep interlobar incisures into four principal =lobes=—=right lateral=, =right central=, =left central=, =left lateral=; the last of these is usually considerably the largest. On the upper part of the right lateral lobe is the =caudate lobe= and its process. That part of the right central lobe which lies below the portal fissure and to the left of the gall-bladder and cystic duct is homologous with the quadrate lobe of man. The parietal surface is extremely convex, its most anterior part reaching to a plane through the lower part of the sixth or seventh intercostal space. The visceral surface is deeply concave; most of it is related to the stomach, for which there is a correspondingly large and deep gastric impression. There is a duodenal impression on the upper part of the right lateral lobe, but no renal impression, as the right kidney does not touch the liver. The fossa for the gall-bladder is mainly on the right central lobe, but also in part on the adjacent surface of the left central lobe. The posterior vena cava enters the dorsal border of the caudate lobe and soon becomes entirely embedded in the gland substance, emerging only at its passage through the diaphragm. The œsophageal notch is large and is occupied mainly by the large right crus of the diaphragm. The right lateral border extends backward to the upper part of the twelfth intercostal space or thirteenth rib. The left lateral border is opposite the eighth rib and intercostal space. The ventral border extends backward about as far as the umbilicus.

There are no lateral ligaments and the falciform ligament disappears early.

The =gall-bladder= is attached in the fossa vesicæ felleæ on the right central lobe, at a considerable distance from the ventral border. The cystic duct joins the hepatic duct at an acute angle immediately after the emergence of the latter from the portal fissure. The =bile-duct= (Ductus choledochus) opens at the papilla duodeni about one or two inches (ca. 2.5 to 5 cm.) from the pylorus.

FIG. 324.—PROJECTION OF VISCERA OF PIG ON BODY-WALL, LEFT SIDE.

_D_, Costal line of diaphragm; _U_, ureter; _V. S._, vesicula
seminalis; _B. g._, bulbo-urethral gland; _P_, penis.
]

FIG. 325.—PROJECTION OF VISCERA OF PIG ON BODY-WALL, RIGHT SIDE.

_D_, Costal attachment of diaphragm; _O_, ovary.
]

Owing to the large amount of interlobular tissue, the =lobules= are mapped out sharply; they are polyhedral in form and are 1 to 2.5 mm. in diameter. For the same reason the gland tissue is much less friable than that of the other animals, from which it is easily distinguished.

THE PANCREAS

The =pancreas= is elongated, and is situated transversely on the dorsal abdominal wall, with its right extremity on the ventral surface of the right kidney, and the left end touching the dorsal end of the spleen and the left kidney. The right portion has two lobes; the posterior lobe bends around the great mesenteric artery and the portal vein; the anterior one is pointed, extends downward and forward about in the median plane, and is attached to the portal fissure and the duodenum. The left portion is prismatic and rests mainly on the left sac of the stomach, its left extremity being included between the left kidney and the dorsal end of the spleen. The pancreatic =duct= passes from the anterior lobe directly through the duodenal wall, opening about five or six inches (ca. 12.5 to 15 cm.) from the pylorus. The interlobular tissue usually contains a good deal of fat.

THE SPLEEN

The =spleen= is long and narrow. Its long axis is nearly dorso-ventral in direction, and is slightly curved to conform to the left part of the greater curvature of the stomach. The dorsal end lies under the vertebral ends of the last three ribs. The ventral end is smaller, reaches to the ventral abdominal wall, and lies against the left lobe of the liver. The visceral surface has a longitudinal ridge on which the hilus is situated; this divides the surface into nearly equal gastric and intestinal areas. The spleen is attached so loosely to the stomach that it may be regarded as being intercalated in the great omentum. In large subjects it may reach a length of 15 to 18 inches (ca. 38 to 45 cm.) and a width of 3 to 4 inches (ca. 8 to 10 cm.).

DIGESTIVE SYSTEM OF THE DOG

THE MOUTH

The size and form of the mouth vary greatly in different breeds, the cavity being in some long and narrow, in others short and wide. The =rima oris= is very extensive, so that the labial commissure is opposite the third or fourth cheek tooth. The =lips= are thin and mobile, and present numerous tactile hairs. The upper lip has a small, central, bare area which forms part of the muzzle, and is marked by a central groove, the philtrum, or (as in the bulldog) a fissure, giving the appearance of harelip. The lateral borders of the lower lip are flaccid and denticulated. The mucous membrane is usually pigmented and forms distinct frena labiorum. The labial glands are small and scanty.

FIG. 326.—TONGUE OF DOG. _a_, Tonsil; _b_, vallate papillæ; _c_,
foliate papilla; _d_, fungiform papillæ; _e_, median groove of
dorsum; _f_, epiglottis; _g_, soft papillæ of root; _h_, middle
glosso-epiglottic fold; _1_, tip; _2_, root. (Ellenberger-Baum,
Anat. d. Haustiere.)
]

The =cheeks= are loose and capacious, and their mucous lining is smooth and more or less pigmented. The =parotid duct= opens usually opposite the third upper cheek tooth. Near the last tooth are the openings of the four or five ducts from the orbital gland.

The =hard palate= is widest between the fourth pair of cheek teeth. It has eight to ten curved ridges on either side of the raphé. Behind the first pair of incisor teeth is the triangular incisive papilla, at which the naso-palatine ducts open. The mucous membrane is usually pigmented.

The =soft palate= is thick except at its margins. In the resting state of the parts it comes in contact with the oral surface of the epiglottis. Between its anterior and posterior pillars on either side is a marked tonsillar sinus, in which an elongated =tonsil= is situated; this is reddish in color, about an inch long, and projects somewhat into the fauces between two folds of mucous membrane.

The =tongue= is wide, thin, and very mobile. It is not pigmented, but has a bright red color. The upper surface is marked by a median groove (Sulcus medianus linguæ), and is thickly beset with short, pointed, filiform papillæ, the free ends of which are directed backward. On the root the papillæ are long and soft; similar papillæ occur on the lateral walls of the isthmus faucium. The fungiform papillæ are small, and are scattered over the dorsum and sides of the tongue, but are absent on the root. There are usually two or three vallate papillæ on either side. Small foliate papillæ are also present. In the inferior part of the tip of the tongue is the =lyssa=, a fusiform cord, composed of fibrous tissue, muscular tissue, and fat. In large dogs it is about two inches (ca. 4 to 5 cm.) long.

FIG. 327.—SKULL OF ADULT ST. BERNARD DOG, SCULPTURED TO SHOW THE
EMBEDDED PARTS OF THE TEETH.

_I1–3_, Incisors; _C_, canines; _P1–4_, premolars; _M1–3_, molars.
]

THE TEETH

The formula for the permanent teeth is:

( 3 1 4 2 )
2 ( I - C - P - M - ) = 42
( 3 1 4 3 )

All of the teeth have short crowns and distinct necks; they erupt rapidly. The crowns are white, being destitute of cement.

The =incisors= are placed almost vertically and close together in the jaw-bones. They do not correspond to an opposing tooth, but rather to parts of two teeth of the other jaw. The crowns are trituberculate, the central projection being the largest. They increase in size from the first to the third. The labial surface is convex, the lingual slightly concave, and marked off from the neck by a =V=-shaped ridge, the cingulum. The roots are flattened transversely. The lower incisors are smaller than the upper ones. One or two supernumerary teeth may be present.

FIG. 328.—BASE OF SKULL OF COCKER SPANIEL.

_I1–3_, Incisors; _C_, canine; _P1–4_, premolars; _M1–2_, molars. Note
the crowding of the canines and premolars on account of the
shortness of the jaw.
]

The =canine teeth= are large, conical, and curved. The upper canine is separated from the corner incisor by an interval into which the lower canine is received when the jaws are closed.[115] The lower canine is close to the corner incisor. The root is about an inch (ca. 2 to 3 cm.) long and is flattened laterally.

The =cheek teeth= are typically ⁶⁄₇, but in brachycephalic breeds they are commonly reduced to ⁵⁄₇, and in extreme cases even to ⅘. The reduction occurs at either end or at both ends of the series.[116] The first tooth appears only once. The fourth tooth of the upper row and the fifth of the lower row are much larger than the rest, and termed =sectorial= or =carnassial= teeth. From these the teeth diminish in size both forward and backward. The upper and lower teeth do not correspond, but rather dovetail. The teeth behind the sectorial ones are tuberculate, _i. e._, have rounded eminences on the masticatory surface. The others are all sectorial in character, _i. e._, have sharp-edged, pointed projections, the middle one being the most prominent. The premolars are laterally compressed, and are separated by intervals from the canines and from each other, except in the brachycephalic breeds. The upper molars have wide, somewhat quadrangular crowns, and three roots. The crown of the upper fourth premolar (Dens sectorius) is divided into two pointed lobes and has an antero-internal tubercle; it has three roots. The crown of the first lower molar (Dens sectorius) is compressed laterally and has two pointed, sharp-edged lobes, behind which are one or two tubercles; it has two roots.

FIG. 329.—DISSECTION OF HEAD OF DOG, SHOWING SALIVARY GLANDS, ETC.

_a_, Ocular muscles; _b_, pterygoideus internus (cut); _c_,
stylo-glossus; _d_, digastricus (cut); _e_, genio-glossus; _f_,
genio-hyoideus; _g_, hyo-glossus; _h_, thyro-pharyngeus; _i_,
crico-pharyngeus; _k_, thyro-hyoideus; _l_, zygomatic process of
temporal (sawn off); _1_, parotid gland; _2_, submaxillary gland;
_3_, posterior part of sublingual gland; _3′_, anterior part of
same; _4_, submaxillary duct; _5_, ductus sublingualis major; _6_,
palatine glands; _7_, orbital or zygomatic gland; _8_, ducts of _7_;
_9_, lacrimal gland. (Ducts colored red.) (After Ellenberger, in
Leisering’s Atlas.)
]

The average periods of eruption are given below.

Tooth Eruption │ Change
│
I_{1} 4 to 5 weeks │4 to 5 months
I_{2} 4 to 5 weeks │ „
I_{3} 4 weeks │ „
C 3 to 4 weeks │4 to 5 months
P_{1} 4 to 5 months│5 to 6 months
P_{2} 4 to 5 weeks │ „
P_{3} 3 to 4 weeks │ „
P_{4} 3 to 4 weeks │ „
M_{1} 4 months │
upper 5 to 6 │
M_{2} months │
lower 4 ½ to │
5 months │
M_{3} 6 to 7 months│

THE SALIVARY GLANDS

The =parotid gland= is small. Its upper part is wide and is divided into two lobes by a deep notch into which the base of the ear is received. The lower end is small and overlaps the submaxillary gland. The =duct= leaves the gland at its anterior border, crosses the masseter muscle, and opens into the mouth opposite the third upper cheek tooth. Small accessory glands (Glandulæ parotidæ accessoriæ) are sometimes found along the course of the duct.

The =submaxillary gland= is usually larger than the parotid. In large dogs it is about two inches (ca. 5 cm.) long and an inch or more (ca. 3 cm.) wide. It is rounded in outline and of a pale yellow color. Its upper part is covered by the parotid, but it is otherwise superficial, and is palpable in the angle of junction of the jugular and external maxillary veins. The =duct= leaves the deep face of the gland, passes along the surface of the digastricus and stylo-glossus, and opens into the mouth near the frenum linguæ on a very indistinct papilla.

FIG. 330.—SAGITTAL SECTION OF PHARYNGEAL AND LARYNGEAL REGIONS OF DOG.

_1_, Œsophagus; _2_, larynx; _3_, mouth cavity; _4_, epiglottis; _5_,
arytenoid cartilage; _6_, soft palate; _6″_, position of soft palate
when raised; _7_, vomer; _8_, base of cranium; _9_, root of tongue;
_a_, _a′_, _a″_, pharynx; _b_, fornix of pharynx; _c_, posterior
wall of pharynx; _d_, floor of pharynx; _e_, pharyngeal surface of
soft palate; _f_, origin of œsophagus; _g_, vestibule of larynx;
_h_, isthmus faucium; _i_, Eustachian opening; _k_, posterior naris.
(After Ellenberger, in Leisering’s Atlas.)
]

The =sublingual gland= is divided into two parts. The posterior part (Glandula sublingualis grandicanalaris) lies on the digastricus muscle in intimate relation with the submaxillary gland, but clearly separable from it after removal of the common fibrous capsule. It has a pointed anterior process. The =duct= (Ductus sublingualis major) accompanies the submaxillary duct, and usually opens beside it, but in some cases joins it. The anterior part (Glandula sublingualis parvicanalaris) is long and narrow; it lies on the stylo-glossus muscle. It has a number (8 to 12) of small ducts (Ductus sublinguales minores), some of which open directly into the mouth, while others join the large duct.

The =orbital= or =zygomatic gland= (Glandula zygomatica) represents the superior molar glands of herbivora. It is prismatic and is situated in the orbital region, between the zygomatic arch and the internal pterygoid muscle, in contact with the periorbita. It has four or five =ducts= (Ductus zygomatici) which open near the last upper cheek tooth; one of them (canal of Nuck) is almost as large as the parotid duct; the others are small.

THE PHARYNX

The fornix is narrow. The orifices of the Eustachian tubes are small and slit-like; the end of the tube causes a rounded projection of the mucous membrane (Torus tubarius). The œsophageal opening is relatively small and is encircled by a fold of the mucous membrane. The hyo-pharyngeus muscle is clearly divided into a kerato-pharyngeus and a chondro-pharyngeus.

THE ŒSOPHAGUS

The =œsophagus= is relatively wide and dilatable except at its origin. The constriction at its origin is termed the =isthmus œsophagi=. The cardiac end is slightly dilated; it joins the stomach under the tenth or eleventh thoracic vertebra, slightly to the left of the median plane. The muscular tissue is striated and consists mainly of two layers of spiral fibers which cross each other; near the cardia, however, the fibers are longitudinal and circular. There are mucous glands and lymph glands in the submucosa throughout.

FIG. 331.—STOMACH OF DOG. PARIETAL SURFACE.

Organ fixed _in situ_ when well filled.
]

THE STOMACH

The =stomach= is relatively large. Its capacity in a dog weighing about 40 pounds is about six to seven pints.

Colin estimates the average capacity at about 3 liters (ca. 6½ pints),
with a range between 0.6 and 8 liters (ca. 1⅓ to 17½ pints). Neumayer
gives the capacity as 100 to 250 c.c. per kilogram of body-weight (ca.
2.7 ounces per pound). The average capacity of the human stomach is
estimated at 35 to 40 ounces—only about one-half of that of a dog of
medium size.

When full it is irregularly pyriform. The left or cardiac part (Corpus ventriculi) is large and rounded, while the right or pyloric part (Pars pylorica) is small and cylindrical. When empty, or nearly so, the left sac is strongly contracted; the pyloric part is not materially affected by variations in the amount of ingesta.

The parietal surface of the full stomach is very extensive, strongly convex, and faces partly forward, but largely to the left. It is related to the liver, the left part of the diaphragm, and the left and ventral abdominal wall as far back as a transverse section through the second or third lumbar vertebra.

The visceral surface is much less extensive and is considerably flattened; it faces chiefly to the right, and is related to the intestine, pancreas, and left kidney.

The upper part of the lesser curvature is nearly straight and vertical, but the lower part forms a deep, narrow angle, due to the fact that the pyloric part is directed sharply forward and upward.

The greater curvature is nearly four times as long as the lesser curvature. In the full stomach it extends considerably behind the left costal arch (having carried the spleen backward); ventrally, it lies on the abdominal wall, almost midway from the xiphoid cartilage to the pubis.

FIG. 332.—STOMACH OF DOG, VISCERAL SURFACE.

Organ fixed _in situ_ when well filled.
]

The left extremity or fundus is large and rounded; it is the most dorsal part of the organ, and lies under the dorsal ends of the eleventh and twelfth ribs.

The pyloric extremity is small and is directed forward and somewhat upward; it lies usually about an inch or two (ca. 3.5 to 5 cm.) below the cardia, a little to the right of the median plane. It is related to the portal fissure of the liver and to the pancreas.

FIG. 333.—EVERTED STOMACH OF DOG FROM WHICH THE MUCOUS MEMBRANE IS
REMOVED.

_O_, Œsophagus; _D_, duodenum; _b_, circular fibers; _c′_, internal
oblique fibers; _c″_, cardiac loop; _c‴_, transition of internal to
external oblique fibers; _d_, fibers uniting branches of cardiac
loop; _p_, pyloric sphincter; _p′_, antral sphincter.
(Ellenberger-Baum, Anat. d. Haustiere.)
]

FIG. 334.—DIAGRAM OF ZONES OF MUCOUS MEMBRANE OF STOMACH OF DOG.
]

The cardia is situated about two to three inches (ca. 5 to 7 cm.) from the left extremity, and is oval; it lies a little to the left of the median plane, below the tenth or eleventh thoracic vertebra.

When empty, or nearly so, the stomach is separated from the ventral abdominal wall by the liver and small intestine, and the greater curvature extends back to the eleventh or twelfth rib. In this state there is not rarely a marked constriction between the right and left portions.

The longitudinal muscular fibers are found on the curvatures and on the pyloric part. The oblique fibers are arranged in two layers: the external layer is a continuation of the longitudinal fibers of the œsophagus onto the body and fundus. The circular layer covers the whole stomach except the fundus, and forms a pyloric sphincter and an antral sphincter. The internal oblique layer is arranged as in the horse, and forms a similar loop-like, cardiac sphincter.

Three regions of the mucous membrane exist. Cardiac glands are found in a very narrow pale zone around the cardiac opening, and also scattered along the lesser curvature. The fundus gland region has a thick, reddish-brown mucous membrane, which lines about two-thirds of the organ. The pyloric mucous membrane is thinner and pale; in the dead subject it is often stained by regurgitated bile.

FIG. 335.—ABDOMINAL VISCERA OF DOG.

Superficial ventral view after removal of the great omentum. From
photograph of formalin-hardened subject.
]

THE INTESTINE

The intestine is short—about five or six times the length of the body.

The =small intestine= has an average length of about thirteen feet (ca. 4 meters). It occupies most of the abdominal cavity behind the liver and stomach. The duodenum begins at the portal fissure and passes backward and upward, at first on the visceral surface of the liver, then in contact with the right flank. Near the pelvis it turns inward and sharply forward, passes along the inner border of the left kidney, and joins the jejunum to the left of the root of the mesentery. The mesoduodenum is given off from the common mesentery; it is a relatively wide fold, except at the terminal part. Here there are two short folds, one of which joins the mesocolon; the other passes on to the terminal part of the colon. The first part of the mesoduodenum contains the right branch of the pancreas. Its root is blended with the mesocolon to form a common mesentery. The remainder of the small intestine forms numerous coils, and is attached by a wide mesentery to the sublumbar region. The =pancreatic= and =bile-ducts= open into the duodenum about two or three inches (ca. 5 to 8 cm.) from the pylorus; the =accessory pancreatic duct= opens an inch or two (ca. 2.5 to 5 cm.) further back. The mucous membrane has very long villi. The duodenal (Brunner’s) glands occur only near the pylorus. Peyer’s patches are numerous (about twenty in young subjects), and begin in the duodenum. They are usually elliptical in outline, but the last one is band-like, reaches to the end of the ileum and is four to sixteen inches (ca. 10 to 40 cm.) long in young dogs (Ellenberger). There is an ileo-cæcal valve.[117] The muscular coat is relatively thick.

FIG. 336.—DEEP DISSECTION OF ABDOMINAL VISCERA OF DOG (FEMALE).

_BL_, Bladder; _B_, body; _C_, _C′_, cornua of uterus; _O_, _O′_,
ovaries; _L. K._, left kidney. The concealed part of the colon is
indicated by dotted lines. (From photograph of formalin-hardened
subject.)
]

The =large intestine= is two to two and a half feet (ca. 60 to 75 cm.) long. Its caliber is about the same as that of the small intestine, and it has neither longitudinal bands nor sacculations.

The =cæcum= is about three or four inches (ca. 8 to 10 cm.) long, and is twisted in a spiral fashion. The twist is maintained by the peritoneum, which attaches it to the ileum. It is situated usually against the middle of the upper part of the right flank, below the duodenum and the right branch of the pancreas.[118]

The =colon= is attached to the sublumbar region by a mesentery, the mesocolon. It presents three parts, which correspond to the ascending, transverse, and descending colon of man. The first or right part (Colon ascendens) is very short. It begins at the junction of the ileum and cæcum, and passes forward along the inner surface of the duodenum and the right branch of the pancreas till it reaches the pyloric part of the stomach; here it turns inward and crosses the median plane, forming the transverse part (Colon transversum). The third or left part (Colon descendens) passes backward in the sublumbar region along the inner border or ventral surface of the left kidney; then inclines toward the median plane and joins the rectum.[119] The caliber of the colon is about the same throughout. It has no bands nor sacculations.

FIG. 337.—CÆCUM OF DOG.

_1_, Ileum; _2_, cæcum; _3_, colon. (From Leisering’s Atlas.)
]

FIG. 338.—TERMINATION OF ILEUM OF DOG.

_a_, Colon; _b_, cæcum; _c_, ileum; _d_, ileo-cæcal opening; _e_,
circular depression around end of ileum. (After P. Schumann.)
]

The mucous membrane of the cæcum contains numerous solitary glands; they are found also in the first part of the colon.

The =rectum= is almost completely covered with peritoneum, the line of peritoneal reflection being under the second or third coccygeal vertebra. At the junction of the rectum and anus the mucous membrane has a stratified, squamous epithelium, and contains the =anal glands=. A small opening on either side leads into two lateral =anal sacs= (Sinus paranales); these are usually about the size of a hazelnut, and contain a dirty gray, fatty substance, which has a peculiar and very unpleasant odor. The skin which lines these pouches contains coil glands. Further back the skin contains large sebaceous glands and peculiar =circumanal glands=.

The retractor ani muscle is extensive. It arises from the shaft of the ilium, the pubis, and the symphysis pelvis, and passes upward and backward to end on the first coccygeal vertebræ and the sphincter ani externus. The retractors, together with the coccygei, form a sort of pelvic diaphragm, analogous to that of man.

THE LIVER

The liver is relatively large, weighing usually about 5 per cent. of the body-weight. It is divided into six or seven =lobes= by fissures which converge at the portal fissure. When the gland is examined in the soft condition, all the lobes are visible on the visceral surface, but usually only four on the parietal surface.

The =left lateral lobe= is the largest, and is oval in outline. The =left central lobe= is much smaller and is prismatic. The =right central lobe= is second in size, and presents a somewhat tongue-shaped quadrate lobe, marked off by the deep fissure in which the gall-bladder lies. The =right lateral lobe= is third in size, and is oval in outline. On its visceral surface is the large =caudate lobe=; this consists of two parts—on the right, the caudate process; on the left, the papillary process or lobe, both often being subdivided by fissures.

FIG. 339.—LIVER OF DOG.

Soft specimen sketched with lobes drawn apart.
]

FIG. 340.—LIVER OF DOG, VISCERAL SURFACE; HARDENED _in situ_.

Left central lobe, gall-bladder, and great part of right central lobe
not visible.
]

When hardened _in situ_ (with the stomach full), the gland presents the following characters:

The visceral surface is marked by a somewhat oblique ridge (corresponding to the lesser curvature of the stomach), which separates two deep cavities. The depression to the left of the ridge is occupied by the body and fundus of the stomach. The depression on the right is smaller, and is occupied by the pyloric part of the stomach, the origin of the duodenum, and the right branch of the pancreas. The gall-bladder is not visible.

When the stomach is empty and contracted, the visceral surface of the
liver is strikingly different. There is then a shallow impression for
the left part of the stomach on the left lobe, and a large convex
area, related to the small intestine and a mass of omentum. The
pyloric and duodenal impressions are not much changed. The
gall-bladder is visible.

The portal fissure is very deep and might well be termed a fossa. It is bounded on the right (externally) by the caudate process, on the left (internally) by the papillary process or lobe, which is directed ventrally. The hepatic artery enters at the dorsal end and the portal vein centrally, while the bile-duct leaves at the ventral part. The left central lobe is not visible, and the right central one is largely concealed.

The parietal surface is extremely convex, being adapted to the diaphragm and the adjacent part of the ventral wall of the abdomen. On it all the lobes except the papillary are completely or partially visible. The gall-bladder is visible in the fissure between the quadrate and chief portions of the right central lobe.

The dorsal border presents a renal impression on its right portion. The posterior vena cava passes downward and forward at first in a deep groove on the caudate lobe, then largely embedded in the parietal surface of the right lateral lobe; it receives two large hepatic veins just before piercing the diaphragm.

The œsophageal notch is large, and is occupied on the right by the thick margin of the hiatus œsophageus. The left border is opposite to the ninth and tenth ribs; it is thin and is marked by numerous indentations. The caudate process extends usually a little beyond the last right rib, between the right kidney and the right branch of the pancreas. The ventral border lies on the ventral abdominal wall a variable distance behind the xiphoid cartilage.

FIG. 341.—SPLEEN OF DOG, VISCERAL SURFACE.

_a_, _a_, Ridge; _b_, ventral end; _c_, dorsal end. (After
Ellenberger, in Leisering’s Atlas.)
]

The =gall-bladder= lies in the fossa vesicæ felleæ, between the two parts of the right central lobe; it does not reach to the ventral border of the liver. The cystic duct joins the hepatic duct at the lower part of the portal fissure, forming with it the bile duct (Ductus choledochus); the latter passes to the right and opens into the duodenum, about two or three inches (ca. 5 to 8 cm.) from the pylorus.

Of the ligaments, the coronary and right lateral are well developed, but the left lateral and falciform are small.

THE PANCREAS

The =pancreas= is =V=-shaped, consisting of two long narrow branches, which meet at an acute angle above the pylorus. The =right branch= extends backward above the first part of the duodenum, below the caudate lobe of the liver and the right kidney, and ends usually a short distance behind the latter. The =left branch= passes inward and backward on the visceral surface of the stomach, and ends under the left kidney. There are two =ducts=: the larger one unites with the bile duct, while the smaller one enters the duodenum an inch or two (ca. 3 to 5 cm.) further back.

THE SPLEEN

FIG. 342.—PROJECTION OF VISCERA OF DOG, (MALE) ON BODY-WALL, LEFT
SIDE.

_A_, _C_, _D_, Apical, cardiac, and diaphragmatic lobes of lung; _P_,
pericardium; _Pr_, prostate. Costal attachment and median line of
diaphragm are dotted.
]

FIG. 343.—PROJECTION OF VISCERA OF DOG (FEMALE) ON BODY-WALL, RIGHT
SIDE.

_A_, _C_, _D_, Apical, cardiac, and diaphragmatic lobes of right lung;
_P._, pericardium; _L. K._, right kidney; _Pan._, right branch of
pancreas; _O_, ovary; _U_, cornu of uterus. Costal attachment and
median line of diaphragm are dotted; also posterior contour of
stomach.
]

The =spleen= is bright red in color in the fresh state. It is somewhat falciform, long, and narrow; the ventral part is the widest. Its position varies in accordance with the fullness of the stomach. When that organ is moderately full the long axis of the spleen corresponds to the direction of the last rib. Its parietal surface is convex and lies largely against the left flank. The visceral surface is concave in its length, and is marked by a longitudinal ridge, on which the vessels and nerves are situated. The dorsal end lies against the left kidney and left crus of the diaphragm. The ventral end is a little further back, and reaches to the ventral wall of the abdomen. It is so loosely attached by the great omentum as to be regarded as an appendage of the latter.

THE RESPIRATORY SYSTEM

The organs of respiration (Apparatus respiratorius) comprise the =nasal cavity=,[120] the =pharynx=, the =larynx=, the =trachea=, the =bronchi=, and the =lungs=. The lungs are the central organs in which the exchange of gases between the blood and the air takes place; the other parts of the system are passages by which the air passes to and from the lungs. The nasal cavity opens externally at the nostrils, and communicates behind with the pharynx through the posterior nares (Choanæ); it contains the peripheral part of the olfactory apparatus, which mediates the sense of smell. The pharynx is a common passage for the air and food—a remnant of the primitive embryonic arrangement; it has been described as a part of the digestive tube. The larynx is a complex valvular apparatus which regulates the volume of air passing through the tract; it is also the chief organ of voice. The trachea, and the bronchi formed by its bifurcation, are permanently open conducting tubes. The thorax, the pleural sacs which it contains, and the muscles which increase or diminish the size of the cavity are also parts of the system. The bones, joints, and muscles of the thorax have already been described.

For topographic reasons two ductless glands, the =thyroid= and the =thymus=, are usually described in this section, although they are in no sense a part of the respiratory system.

RESPIRATORY SYSTEM OF THE HORSE

THE NASAL CAVITY

The =nasal cavity= (Cavum nasi), the first segment of the respiratory tract, is a long, somewhat cylindrical passage, inclosed by all the facial bones except the mandible and hyoid. It is separated from the mouth ventrally by the palate. It opens externally at the nostrils, and communicates posteriorly with the pharynx through the posterior nares.

The =nostrils= or =anterior nares= (Nares) are somewhat oval in outline, and are placed obliquely, so that they are closer together below than above. They are bounded by two =alæ= or =wings= (Alæ nasi), which meet above and below, forming the =commissures=. The outer ala is concave; the inner one is convex above, concave below. The upper commissure is narrow, the lower one wide and rounded. If the finger is passed into the nostril at the upper commissure, it enters the so-called =false nostril=, which leads backward to a blind cutaneous pouch. This cul-de-sac, the =diverticulum nasi=, extends backward to the angle of junction of the nasal bone and the nasal process of the premaxilla. The posterior part of the diverticulum is blind, but the false nostril communicates below and internally with the vestibule of the nasal cavity. The external opening of the =naso-lacrimal duct= is seen when the nostril is dilated; it is situated on the floor of the vestibule, perforating the skin close to its junction with the mucous membrane, about two inches (ca. 5 cm.) from the lower commissure. (It is not rare to find one or two accessory orifices further back.)

=Structure.=—The =skin= around the nostrils presents long tactile hairs as well as the ordinary ones. It is continued around the alæ and lines the vestibule. In the posterior part of the diverticulum the skin is thin and usually black, and is covered with exceedingly fine hairs; it is provided with numerous sebaceous glands. The inner wing is supported by the =alar cartilage= (Cartilago alaris), which is shaped somewhat like a comma, the convex margin being direct medially. The cartilages are attached by fibrous tissue to the extremity of the septal cartilage. Each consists of an upper, quadrilateral curved plate, the =lamina=, and a narrow =cornu= which curves downward and outward, supporting the inner wing and the lower commissure, but not entering into the formation of the outer wing. The lamina causes the projection of the upper part of the internal wing, from which the thick =alar fold= (Plica alaris) passes backward along the external wall of the nasal cavity to cover the cartilaginous prolongation of the inferior turbinal bone. This fold separates the true from the false nostril. The extremity of the cornu usually causes a slight projection of the skin a short distance behind and below the lower commissure.

FIG. 344.—NOSTRILS OF HORSE.

_a_, Diverticulum nasi (shown as if distended); _b_, philtrum; _c_,
false nostril; _d_, true nostril; _e′_, prominence caused by lamina
of alar cartilage; _e″_, prominence over cornu of same; _f_,
external wing of nostril; _g_, internal wing; _h_, lower commissure;
_i_, orifice of naso-lacrimal duct. (Ellenberger-Baum, Anat. d.
Haustiere.)
]

FIG. 345.—CROSS-SECTION OF NARIAL REGION OF HORSE.

_1_, Nasal bone; _2_, tendon of levator labii superioris proprius;
_3_, external nasal nerve; _4_, nasal diverticulum; _5_, outer wall
of _4_; _6_, dilatator naris inferior; _7_, levator nasolabialis +
dilatator naris lateralis; _8_, maxilla and premaxilla; _9_,
naso-lacrimal duct; _10_, _11_, chief branches of superior labial
nerve, with superior labial artery at their inner side; _12_, nasal
cavity; _13_, superior turbinal fold; _14_, inferior turbinal fold,
inclosing cartilaginous prolongation of inferior turbinal bone;
_15_, venous plexus; _16_, septum nasi; _16′_, parietal cartilage;
_17_, vomero-nasal organ (of Jacobson); _18_, venous plexus of
palate; _19_, buccinator; _20_, skin; _23_, anterior nasal nerve;
_24_, palatine process of premaxilla. (After Ellenberger-Baum, Top.
Anat. d. Pferdes.)
]

The muscles of the nostrils have been described.

=Blood-supply.=—Palato-labial, superior labial, and lateral nasal arteries.

=Nerve-supply.=—Trigeminal and facial nerves.

The nasal cavity is divided into two symmetrical halves by the median =septum nasi=. The osseous part of the septum (Septum osseum) is formed behind by the perpendicular plate of the ethmoid and below by the vomer. A few ridges on the former correspond to the ethmoidal meatuses. The major part of it, however, is formed by the =septal cartilage= (Cartilago septi nasi). The surfaces of the cartilage are marked by faint grooves for the vessels and nerves which course over it. The =dorsal border= is attached along the frontal and nasal sutures, and extends beyond the apices of the nasal bones about two inches (ca. 5 cm.). From this border a thin, narrow plate, the =parietal cartilage= (Cartilago parietalis) curves outward for a short distance on either side. Near the nostrils these plates are somewhat wider, partially making good the defect in the bony roof of the cavity in this situation. The =ventral border= is thick and rounded; it lies chiefly in the groove of the vomer, but anteriorly it occupies the space between the premaxillary bones. The =posterior border= is continuous with the perpendicular plate of the ethmoid bone.[121] The alar cartilages are attached to the anterior extremity by fibrous tissue in such a manner as to allow very free movement—in fact, an actual joint may be found. The ventral part of the cartilage is about half an inch (ca. 1 cm.), the middle part about one-tenth of an inch (ca. 2.5 mm.), and the dorsal part about a quarter to a third of an inch (ca. 6 to 8 mm.) in thickness.

FIG. 346.—NASAL CARTILAGES OF HORSE, DORSAL VIEW. (After
Ellenberger-Baum, Anat. für Künstler.)
]

The two =turbinal bones= (Conchæ nasales) project from the lateral wall, and divide the outer part of the cavity into three =meatuses=—superior, middle, and inferior.

The =superior meatus= (Meatus nasi dorsalis) is a narrow passage, bounded dorsally by the roof of the cavity, and ventrally by the superior turbinal bone; its posterior end is closed by the junction of the inner plate of the frontal bone with the cribriform plate and lateral mass of the ethmoid. It transmits air to the upper part of the olfactory region.

FIG. 347.—NASAL CAVITY OF HORSE, SAGITTAL SECTION WITH SEPTUM REMOVED.

_1_, Superior turbinal fold; _2_, alar fold, containing cartilaginous
prolongation of inferior turbinal; _3_, arrow pointing to
naso-maxillary fissure; _4_, cranial cavity; _5_, sphenoidal sinus;
_6_, hamulus of pterygoid bone. The olfactory mucous membrane is
shaded. Dotted lines indicate anterior limit of uncoiled parts of
turbinal bones, which inclose parts of frontal and maxillary
sinuses.
]

The =middle meatus= (Meatus nasi medius) is between the two turbinal bones. It is somewhat larger than the superior meatus, and does not extend back so far; it ends near the great ethmoturbinal and the ethmoidal meatuses. In its posterior part is found the extremely narrow, slit-like interval by which the maxillary sinus communicates with the nasal cavity—the =naso-maxillary fissure= (Aditus naso-maxillaris). The fissure is not visible from the nasal side, being concealed by the overhanging superior turbinal. A fine flexible probe, passed outward and somewhat backward between the turbinal bones, enters the maxillary sinus; if introduced a little further in the same direction, it usually passes through the orifice of communication between the maxillary and the frontal sinus and enters the latter. A small part of the fissure usually brings the anterior division of the maxillary sinus into communication with the nasal cavity. The spaces inclosed by the coiled portions of the turbinals also open into the middle meatus. This passage may be characterized as the sinus-meatus, but it also conducts air to the olfactory region.

FIG. 348.—CROSS-SECTION OF HEAD OF HORSE. THE SECTION PASSES THROUGH
THE INTERNAL CANTHI.

_1_, Mylo-hyoideus; _2_, stylo-glossus; _3_, hyo-glossus; _4_,
genio-glossus; _5_, lingual process of hyoid bone. An arrow points
to the naso-maxillary fissure.
]

The =inferior meatus= (Meatus nasi ventralis) is situated between the inferior turbinal and the floor of the cavity. It is much larger than the other two, and is the direct passage between the nostrils and the pharynx.

The =common meatus= (Meatus nasi communis) is situated between the septum and the turbinals, and is continuous externally with the other meatuses. It is very narrow dorsally, but widens ventrally.

The lateral masses of the ethmoid bone project forward into the posterior part (fundus) of the nasal cavity (Fig. 347). Between the ethmoturbinals of which each mass is composed there are three principal and numerous small passages, the =ethmoidal meatuses= (Meatus ethmoidales).

The =posterior nares= (Choanæ) are two elliptical orifices by which the nasal cavity and pharynx communicate. They are in the same plane as the floor of the nasal cavity, and are separated from each other by the vomer. They are, taken together, about two inches (ca. 5 cm.) wide and three to four inches (ca. 8 to 10 cm.) long.

The =nasal mucous membrane= (Membrana mucosa nasi) is thick, highly vascular, and is, in general, firmly attached to the underlying periosteum and perichondrium. It is continuous in front with the skin which lines the nostrils, and behind with the mucous membrane of the pharynx. It is also continuous at the naso-maxillary fissure with the very thin and only slightly vascular mucous membrane which lines the sinuses. In the anterior part of the cavity it forms prominent thick folds on the lateral wall, which extend from the turbinal bones to the nostril. There are usually two =superior turbinal folds= which unite anteriorly. The upper one incloses a thin plate of cartilage which is continuous with the superior turbinal bone. The =inferior turbinal fold= is curved, and incloses an ᔕ-shaped cartilaginous plate which prolongs the inferior turbinal bone; this fold is continuous with the alar fold of the nostril, and forms with it the upper margin of the entrance from the true nostril to the nasal cavity. Below this there is a rounded ridge produced by the nasal process of the premaxilla. The mucous membrane of the greater part of the cavity (Regio respiratoria) is red in color, and is covered with a stratified ciliated epithelium. It contains numerous acinous =nasal glands= (Glandulæ nasales). The submucosa contains rich venous plexuses which form in certain situations a sort of cavernous tissue (Plexus cavernosus), composed of several strata of freely anastomosing veins, between which there are unstriped muscle-fibers. This arrangement is most marked in the turbinal folds, on the lower part of the inferior turbinal bone, and the lower part of the septum. On the posterior part of the lateral masses of the ethmoid and the adjacent part of the superior turbinal and the septum (Regio olfactoria) the mucous membrane is brownish-yellow in color and thicker; it contains the olfactory nerve-endings in a special non-ciliated epithelium. In it are numerous =olfactory glands= (Glandulæ olfactoriæ); these are long, tubular, and often branched.

The =vomero-nasal organ= of Jacobson (Organon vomeronasale) is situated on the floor of the nasal cavity, on either side of the ventral border of the septum (Fig. 345.) It consists of a tubular cartilage lined with mucous membrane, to which fibers of the olfactory nerve may be traced. Its anterior part communicates with the nasal cavity by a slit-like orifice.

The =paranasal sinuses= are described in the Osteology.

THE LARYNX

The =larynx= is a short tube which connects the pharynx and trachea. It is a complex valvular apparatus, which regulates the volume of air in respiration, prevents aspiration of foreign bodies, and is the chief organ of voice.

It is situated partly between the internal pterygoid muscles, partly in the neck between the parotid glands.[122] Its long axis is practically horizontal when the head and neck are extended (Fig. 254).

It is related dorsally to the pharynx and the origin of the œsophagus. Ventrally it is covered by the skin, fascia, and sterno-hyoid and omo-hyoid muscles. Laterally it is related to the parotid and submaxillary glands and to the internal pterygoid, stylo-maxillaris, digastricus, stylo-hyoid, and pharyngeal constrictor muscles.

It is attached to the body and thyroid cornua of the hyoid bone, and thus indirectly to the base of the cranium.

Its cavity communicates anteriorly with the pharynx and (potentially) with the mouth; posteriorly, with the trachea.

The wall of the larynx consists of a framework of cartilages, connected by joints and ligaments or membranes, and moved by extrinsic and intrinsic muscles. The cavity is lined with mucous membrane.

CARTILAGES OF THE LARYNX

There are three single cartilages and one pair; the single cartilages are the cricoid, thyroid, and epiglottis; the arytenoid cartilages are paired.

FIG. 349.—LARYNGEAL CARTILAGES OF HORSE, RIGHT VIEW.
]

The =cricoid cartilage= (Cartilago cricoidea) is shaped like a signet ring. The dorsal part is a broad, thick, quadrilateral plate termed the =lamina=; the outer (dorsal) surface of this is marked by a median ridge separating two shallow cavities, which give attachment to the posterior crico-arytenoid muscles. On either side of these depressions are two articular facets. The =anterior facets= (Facies articulares arytænoideæ) are placed at the anterior border, are oval and convex, and articulate with the arytenoid cartilages. The =posterior facets= (Facies articulares thyreoideæ) are situated on the front of a ridge, a short distance from the posterior border; they are concave, and articulate with the posterior cornua of the thyroid cartilage. The ventral and lateral parts of the ring are formed by a curved band, called the arch (Arcus), which is narrowest below. The lateral surfaces of the arch are grooved for the crico-thyroid muscle. The anterior border of the lamina is thick and slightly concave; the posterior border is thin and irregularly notched. The anterior border of the arch is concave ventrally and gives attachment to the crico-thyroid membrane; laterally it is thicker and gives attachment to the crico-arytenoideus lateralis muscle. The posterior border is attached to the first ring of the trachea by an elastic membrane. The inner surface is smooth and is covered with mucous membrane.

FIG. 350.—CRICOID CARTILAGE OF HORSE, LEFT LATERAL VIEW.
]

The =thyroid cartilage= (Cartilago thyreoidea) consists of a median thickened portion, termed the body, and two lateral laminæ or wings. The =body= forms ventrally a slight prominence (Prominentia laryngea), which can be felt, but is not visible in the living subject; it supports the base of the epiglottis, which is attached to it by an elastic ligament. The =wings= or =laminæ= spring from the body on either side and form a large part of the lateral wall of the larynx. Each is a rhomboid plate, presenting a slightly convex outer surface, which is divided into two areas by an oblique line (Linea obliqua), on which the thyro-hyoid and thyro-pharyngeus muscles meet. The superior (dorsal) border is nearly straight; it gives attachment to the pharyngeal fascia and the palato-pharyngeus muscle, and bears a cornu at each end. The =anterior cornu= (Cornu orale) is attached to the thyroid cornu of the hyoid bone; below it is a notch, which is converted into a foramen (Foramen thyreoideum) by a fibrous band, and transmits the superior laryngeal nerve to the interior of the larynx. The =posterior cornu= (Cornu caudale) articulates with the cricoid cartilage. The inferior (ventral) border is fused with the body in front; behind, it diverges from its fellow to inclose a triangular space (Incisura thyreoidea caudalis), which is occupied by the crico-thyroid membrane. The anterior border is slightly convex, and is attached to the hyoid bone by the thyro-hyoid membrane. The posterior border overlaps the arch of the cricoid cartilage, and gives attachment to the crico-thyroid muscle. The inner surface is concave, and is covered by mucous membrane to a small extent.

FIG. 351.—THYROID CARTILAGE OF HORSE, VENTRAL VIEW.
]

The =epiglottis= (Cartilago epiglottica) is situated above the body of the thyroid cartilage and curves toward the root of the tongue. It is shaped somewhat like a pointed ovate leaf, and presents two surfaces, two borders, a base, and an apex. The =oral= (or anterior) =surface= is concave in its length, convex transversely; the =pharyngeal= (or posterior) =surface= has the reverse configuration. The =borders= are thin, denticulated, and somewhat everted. The =base= is thick, and is attached to the dorsal surface of the body of the thyroid cartilage by elastic tissue. From each side of it a cartilaginous bar projects backward; these processes correspond to the cuneiform cartilages of man. The =apex= is pointed and curved ventrally. Nearly all of the epiglottis is covered with mucous membrane. It lies sometimes in front of, sometimes behind, the soft palate.

FIG. 352.—EPIGLOTTIS OF HORSE, RIGHT LATERAL VIEW.
]

The =arytenoid cartilages= (Cartilagines arytenoideæ) are situated on either side, in front of the cricoid, and partly between the alæ of the thyroid cartilage. They are somewhat pyramidal in form, and may be described as having two surfaces, two borders, a base, and an apex. The =internal surface= is concavo-convex, smooth, and covered by mucous membrane. The =external surface= is divided by a ridge into two areas: the lower flat area faces outward, and is covered by the thyro-arytenoid and lateral crico-arytenoid muscles; the upper concave area faces chiefly forward, and is covered by the transverse arytenoid muscle. The =anterior border= is convex, and at the base of the cartilage forms a projection termed the =vocal process= (Processus vocalis); it is so named because it gives attachment to the vocal or thyro-arytenoid ligament, which forms the basis of the true vocal cord. The =posterior border= is straight, and extends from the vocal process to the facet on the base. The =dorsal border= forms a deep notch with the apex. The =base= is concave and faces chiefly backward; it presents an oval, concave facet for articulation with the anterior border of the lamina of the cricoid cartilage. The external angle of the base forms a strong eminence termed the =muscular process= (Processus muscularis), which gives attachment to the crico-arytenoid muscles. The internal angle is attached to its fellow by the transverse arytenoid ligament. The =apex= (Cartilago corniculata) curves upward and backward, forming with its fellow the pitcher-shaped lip, from which the cartilages derive their name.

FIG. 353.—LEFT ARYTENOID CARTILAGE OF HORSE, INNER VIEW.
]

The cricoid and thyroid cartilages, and the greater part of the arytenoid cartilages, consist of hyaline cartilage. The apices and vocal processes of the arytenoid cartilages and the epiglottis (including the cuneiform processes) consist of elastic cartilage; they show no tendency toward ossification at any age. The thyroid and cricoid cartilages regularly undergo considerable ossification; the process begins in the body of the thyroid, and often involves the greater part of the cartilage.

JOINTS, LIGAMENTS, AND MEMBRANES OF THE LARYNX

The =crico-thyroid joints= (Articulationes crico-thyreoideæ) are diarthroses formed by the apposition of the convex facets on the ends of the posterior cornua of the thyroid cartilage with corresponding facets on the sides of the cricoid cartilage. The =capsule= is thin, but is strengthened by accessory bands dorsally, externally, and internally. The movements are chiefly rotation around a transverse axis passing through the centers of the two joints.

The =crico-arytenoid joints= (Articulationes crico-arytænoidea) are also diarthrodial. They are formed by the apposition of the convex facets on the anterior border of the cricoid cartilages with the concave facets on the bases of the arytenoid cartilages. Each has a very thin, loose =capsule=, strengthened by accessory bundles dorsally and internally. The movements are gliding and rotation. In the former the arytenoid cartilage glides inward or outward; in the latter it rotates around a longitudinal axis, so that the vocal process swings outward or inward.

The =thyro-hyoid joints= (Articulationes hyo-thyreoideæ) are formed by the anterior cornua of the thyroid cartilage and the extremities of the thyroid cornua of the hyoid bone. They permit rotation around a transverse axis passing through the two joints.

The =crico-thyroid membrane= (Ligamentum crico-thyreoideum) occupies the thyroid notch and extends backward to the arch of the cricoid cartilage. It is triangular in form, and is attached by its base to the anterior border of the arch of the cricoid cartilage, while its borders are attached to the margins of the thyroid notch. It is strong, tightly stretched, and composed largely of elastic tissue. It is reinforced ventrally by longitudinal fibers, dorsally by fibers which stretch across the thyroid notch.

The =thyro-hyoid membrane= (Membrana hyo-thyreoidea) connects the anterior border of the thyroid cartilage with the body and thyroid cornua of the hyoid bone.

The =hyo-epiglottic ligament= (Ligamentum hyo-epiglotticum) attaches the lower part of the oral surface of the epiglottis to the body of the hyoid bone. It forms an elastic sheath for the hyo-epiglottic muscle. Ventrally it blends with the thyro-hyoid membrane, and dorsally it is not well defined.

The =thyro-epiglottic ligament= (Ligamentum thyreo-epiglotticum) is strong and thick; it is composed chiefly of elastic tissue. It connects the base of the epiglottis with the body and the adjacent inner surface of the laminæ of the thyroid cartilage. Other fibers attach the cuneiform processes somewhat loosely to the thyroid laminæ.

The =transverse arytenoid ligament= (Ligamentum arytænoideum transversum) is a slender band which connects the inner angles of the bases of the arytenoid cartilages.

The =vocal= (or inferior thyro-arytenoid) =ligament= (Ligamentum vocale) forms the basis of the true vocal cord (Fig. 254). It is elastic and is attached ventrally to the body of the thyroid cartilage and the crico-thyroid membrane close to its fellow of the opposite side, and ends dorsally on the processus vocalis of the arytenoid cartilage.

The =ventricular= (or superior thyro-arytenoid) =ligament= (Ligamentum ventriculare) is included in the false vocal cord. It consists of loosely arranged bundles which arise chiefly from the cuneiform process and end on the outer surface of the processus vocalis and adjacent part of the arytenoid cartilage.

The =crico-tracheal membrane= (Ligamentum crico-tracheale) connects the cricoid cartilage with the first ring of the trachea.

MUSCLES OF THE LARYNX

A. EXTRINSIC MUSCLES

The =sterno-thyro-hyoideus= is described on p. 226.

The =thyro-hyoideus= (M. hyo-thyreoideus) is a flat, quadrilateral muscle, which lies on the outer surface of the thyroid lamina and the lateral part of the thyro-hyoid membrane. It arises from the thyroid cornu of the hyoid bone, and ends on the oblique line on the lamina of the thyroid cartilage. Its action is to draw the larynx toward the root of the tongue.