Part 21

The size of the great omental sac is increased by the =lesser omentum=. This is a double sheet of peritoneum which stretches horizontally from the liver to the duodenum and the lesser curvature of the stomach. It covers ventrad the caudate lobe of the liver. Its right border is at the foramen epiploicum. The part of it which stretches to the duodenum is called the =duodenohepatic= ligament and contains the bile-ducts and the portal vein. The part that stretches to the stomach is called the =gastrohepatic= ligament. The peritoneum covers the caudal and cranial surfaces of the liver and is reflected to the adjacent parts to form the ligaments of the liver.

The =suspensory= ligament of the liver passes from the caudal surface of the diaphragm and the median ventral line for about one or two centimeters caudad of the diaphragm, to the liver, and extends between its two halves. The =ligamentum teres= or round ligament is the thickened free caudal border of the suspensory ligament. It is the remains of the fœtal umbilical vein. From the dorsal border of the liver the peritoneum which covers its cranial surface turns ventrad onto the caudal surface of the diaphragm, while that which covers its caudal surface turns dorsad onto the caudal surface of the diaphragm to reach the dorsal body wall. Between these two sheets a small linear part of the surface of the liver is closely applied to the diaphragm without intervening peritoneum. The two sheets which bound this area constitute the =coronary= ligament of the liver. This ligament is broader where it passes from the left lateral lobe to the diaphragm and is called the =triangular= ligament. (There is perhaps a corresponding right triangular ligament, from the cranial division of the right lateral lobe.) The caudal division of the right lateral lobe is held to the kidney of that side by the =hepatorenal= ligament.

II. THE ALIMENTARY CANAL. APPARATUS DIGESTORIUS.

The alimentary canal may be divided into mouth, pharynx, œsophagus, stomach, small intestine, and large intestine. With these are associated certain accessory structures,--the salivary glands, the liver, and the pancreas. The spleen, though not belonging to the digestive system, is usually described in connection with it. The respiratory organs are almost throughout in close relation with the organs of the digestive system.

1. =The Mouth. Cavum oris.=--The mouth cavity extends from the lips to the pharynx. It is narrower toward the lips, broadens caudad as far back as the last teeth, then becomes narrowed to form the =isthmus faucium=, by which it communicates with the pharynx. The mouth cavity is divisible into the =vestibule= of the mouth (=vestibulum oris=), which comprises that part outside the jaws proper, bounded externally by the lips and cheeks, and the =mouth cavity proper= (=cavum oris proprium=), which lies within the teeth. That portion of the vestibule which is bounded by the cheeks is sometimes farther distinguished as the =buccal cavity=. The entire mouth cavity (except the teeth) is lined by the mucous membrane or =mucosa=.

The =lips= (=labia oris=) are thick folds of skin bounding the entrance to the mouth cavity. The outer surface is covered with hair; the inner surface is covered with the mucous membrane. The upper lip is marked in the median line by a deep external groove which extends upward to the septum of the nose. Along the inner surface of this groove the lip is closely united to the jaw by a thick fold, the =frenulum= of the upper lip. For some distance on each side of the frenulum the inner surface of the lip bears numerous large papillæ. The =lower= lip is also united to the jaw by a frenulum in the median line; it is again united to the jaw just caudad of the canine tooth, in the space between the latter and the first premolar. Caudad the two lips pass into each other (forming the =commissura labiorum=), and unite with the cheek. The muscles of the lips have been described (page 105).

The =cheeks= (=buccæ=) in the cat are comparatively thin and small, extending from the lips caudad to the ramus of the mandible. The outer surface is covered with hair; the inner surface is smooth and somewhat folded. The buccal cavity is rather small. On the inner surface of the cheek open the duct of the parotid (Steno’s duct), the ducts of the molar gland, and that of the infraorbital gland.

The roof of the mouth cavity is formed by the hard and soft palates. The hard palate (=palatum durum=) forms the cranial part of the roof; it is supported by the palatal plates of the maxillary and palatine bones. The mucosa of the hard palate is elevated to form seven or eight curved transverse ridges, which are concave caudad. Between the ridges are rows of papillæ. In front of the most cranial ridge is a papilla in the middle line, and at each side of the papilla is the opening of a small duct (the =incisive duct= or =Stenson’s duct=), which leads dorsad through the incisive foramen to the =vomeronasal= organ (or =organ of Jacobson=), which lies on the floor of the nasal cavity. Caudad of the hard palate the roof of the mouth is formed by the soft palate or =velum palatinum=, described below.

The floor of the mouth cavity is formed chiefly by the tongue, which extends as far caudad as the isthmus faucium. Ventrad of the free edge of the tongue the mucosa forms a prominent median vertical fold which unites the tongue with the floor of the mouth beneath it; this fold is the frenulum linguæ. On each side of the median line at the cranial border of the floor of the mouth is a prominent papilla, at the apex of which open the ducts of the submaxillary and sublingual glands, the former on the lateral side of the apex, the latter on the medial side.

The sides of the mouth cavity are formed by the teeth and the gums, covering the alveolar borders of the mandible, maxillaries, and premaxillaries.

The mouth cavity presents further for examination the =glands=, the =teeth=, the =tongue=, and the =soft palate=.

THE GLANDS OF THE MOUTH (GLANDULÆ ORIS).--There are five pairs of salivary glands which open into the mouth cavity.

1. The =parotid gland= (=glandula parotis=) (Fig. 65, 1; Fig. 131, 10) is flattened, rather finely lobulated, and lies ventrad of the external auditory meatus and beneath the dermal muscles. Its cranial border follows the caudal border of the masseter muscle and overlies it somewhat; its caudal border is about three centimeters caudad of the border of the masseter. Its borders are unevenly lobed. The parotid duct (=ductus parotideus=: frequently called =Stenon’s= or =Steno’s duct=) is formed by the union of several smaller ducts near the ventral end of the cranial border of the gland. It passes craniad imbedded in the fascia covering the masseter. At the cranial border of the masseter it turns inward and lies close against the mucous membrane of the mouth, so that from the inside of the mouth it appears as a white ridge on the mucosa. It opens on the inside of the cheek opposite the most prominent cusp of the last premolar tooth. Along the course of the parotid duct in some cases one or more small =accessory parotid glands= are found.

2. The =submaxillary gland= (=glandula submaxillaris=) (Fig. 65, 2, page 109, and Fig. 131, 11) is approximately kidney-shaped. Its surface is nearly smooth, the lobulations not being apparent externally. It lies ventrad of the parotid, at the caudal edge of the masseter muscle, just caudad of the angular process of the mandible. The posterior facial vein (Fig. 131, =b=) crosses its outer surface, and its cranioventral border is hidden by two lymphatic glands (Fig. 131, 12) lying at the sides of the anterior facial vein. The =submaxillary duct= (=ductus submaxillaris=, frequently called =Wharton’s duct=) leaves the inner surface of the gland and passes beneath the digastric and mylohyoid muscles and against the outer surface of the styloglossus. From the point where the styloglossus passes into the tongue the duct continues craniad close against the oral mucosa and parallel to the mandibula. It is accompanied by the duct of the sublingual, which lies at first dorsad of it and then mediad. It opens at the apex of the prominent papilla which lies at the side of the middle line at the cranial end of the floor of the mouth.

3. The =sublingual gland= is elongated and conical in form, with its base against the submaxillary, of which it appears to be a continuation. It stretches along the submaxillary duct for about one and one-half centimeters, lying between the masseter and digastric muscles. The =sublingual= duct leaves its ventral side, passes close to the submaxillary duct, at first dorsad and then mediad of it, and opens on the medial side of the apex of the same papilla with the submaxillary duct.

4. =The molar gland= (=glandula molaris=) (Fig. 65, 9) lies between the orbicularis oris and the mucosa of the lower lip. It stretches from the cranial border of the masseter to a point between the first premolar and the canine. It is flat, broad caudad, and ends in a point craniad. It has several ducts which pass straight through the cheek and open on the mucous surface of the mouth.

5. The =infraorbital= (or =orbital=) =gland= lies in the lateral part of the orbit on its ventral floor. It is ovoid and about one and one-half centimeters long and one-third as thick. Its ventral end rests against the mucosa of the mouth just caudad of the molar tooth. Its duct leaves the ventral end and opens into the mouth at a point about three millimeters caudad of the molar tooth.

THE TEETH. DENTES.--The adult cat has thirty teeth, fourteen in the lower jaw and sixteen in the upper jaw. There are twelve =incisors=, four =canines=, ten =premolars=, and four =molars=. The tooth formula for the cat is then

3 - 3 1 - 1 3 - 3 1 - 1 _i_-----, _c_-----, _pm_-----, _m_-----. 3 - 3 1 - 1 2 - 2 1 - 1

The teeth are implanted in the alveolar borders of the premaxillaries, maxillaries, and mandible. In each tooth can be distinguished the =root=, imbedded in the socket of the bone, the =crown=, which projects above the gums, and a narrow =neck= connecting the two. The root is composed of one or more separate fangs; the crown bears one or more points or cusps.

At the cranial end of each jaw are six =incisor= teeth (Figs. 93 and 94, _a_). These are imbedded in the alveolar borders of the premaxillaries and the mandible. The incisor teeth are small, with a crown bearing a sharp edge which is notched so as to form three minute cusps. The root of each has a single fang. The lateral incisors are the largest in each jaw, and those of the upper jaw are larger than those of the lower.

Caudad of the incisors, and in the upper jaw separated from them by a slight interval, are the =canines= (_b_), two in each jaw. These are long, strong, pointed teeth, deeply imbedded in the mandible and maxillaries, their large sockets causing a rounded swelling on the external surface of the bones. Each has a single fang and a single cusp. When the mouth is closed the upper canines lie laterocaudad of the lower ones.

Caudad of the canines there is in each jaw a considerable interval free from teeth: this is called the =diastema=. Caudad of the diastema are the =premolar= teeth, three pairs (_c_, _d_, _e_) in the upper jaw and two pairs (_c_, _d_) in the lower jaw. These teeth are compressed sideways, and those of the lower jaw fit inside of those of the upper jaw. In the =upper= jaw (Fig. 93) the first premolar (_c_) is small and usually has but a single cusp and a single fang, though occasionally there is a small supplementary cusp and fang. The second premolar is larger (_d_); it has a large central cusp, with a single smaller cranial cusp and two small caudal cusps, making four in all. This tooth has two fangs. The third premolar (_e_) is the largest tooth in the jaws; it has three large cusps in longitudinal series and a small cusp lying on the medial side of the first one in the row. Its root has three fangs. The =molar= tooth (_f_) of the upper jaw is small and lies caudomediad of the last premolar. It has two small cusps and two fangs.

In the =lower= jaw (Fig. 94) the two premolars (_c_ and _d_) are similar, the caudal one being a little larger. Each has four cusps; a single large one, a small one craniad of this, and two small ones caudad of it. Each has two fangs. The single molar (_f_) is the largest tooth of the lower jaw; it has two large cusps and two fangs.

=The Deciduous Teeth.=--At birth the cat has no teeth. There appears later a set of twenty-six teeth: twelve incisors, four canines, and ten molars (six in the upper jaw and four in the lower). These teeth are later replaced by the permanent ones above described. The deciduous teeth of the cat are fully described by Jayne (“Mammalian Anatomy,” vol. 1. p. 319), where also an account is given of the order of appearance of the teeth.

THE TONGUE. LINGUA (Fig. 95).--The =tongue= is a muscular organ covered with mucous membrane; in life it is very mobile. It is an elongated organ, flat above, broadest in the middle, and very slightly narrowed at each end. It extends from the incisor teeth to the isthmus faucium and nearly fills the mouth cavity. The caudal third of the tongue forms the floor of the mouth cavity, so that the tongue has here no ventral surface, but is directly continuous with underlying organs. It is in this region that the extrinsic muscles of the tongue (except the genioglossus) enter it. The cranial two-thirds of the tongue is partly or entirely free from the floor of the mouth, the cranial one-third being completely free and movable. In about the middle third the ventral surface of the tongue is held to the floor of the mouth by the fold known as the =frenulum linguæ=. The frenulum contains parts of the two genioglossus muscles, which enter the tongue through it. The ventral surface and lateral borders of the tongue are smooth, soft, and free from papillæ. The dorsal surface is raised into papillæ of various kinds, and has a slight median longitudinal furrow. The caudal part of the dorsal surface is softer, redder, and marked with papillæ of a different kind from those of the rest of the tongue. From the caudal end a small median vertical fold, the =frenulum= (or =plica=) =glossoepiglottica= passes from the dorsal surface of the tongue to the cranial surface of the epiglottis.

The =papillæ= of the tongue are of three kinds. 1. The very numerous =filiform papillæ= (_a_) (=papillæ filiformes=); many of them are horny and tooth-like, with points turned caudad. These are most numerous at the middle of the free end of the tongue. 2. The =fungiform papillæ= (_b_) (=papillæ fungiformes=) are found scattered over the surface of the middle of the tongue caudad of the large filiform papillæ. They are enlarged at their free ends. There is a prominent row of very large ones (_b′_) opposite the circumvallate papillæ at the borders of the tongue. 3. The =circumvallate papillæ= (_c_) (=papillæ vallatæ=) are blunt and each is surrounded by a trench which is bounded in turn by a raised wall. They are in two rows of two or three each, which converge near the base of the tongue so as to form a V with the apex directed caudad.

MUSCLES OF THE TONGUE (Fig. 96).--=M. genioglossus= (_f_) passes from the symphysis of the lower jaw into the tongue and lies beneath (dorsad of) the geniohyoid (_g_).

_Origin_ from the medial surface of the mandible near the symphysis and dorsad of the origin of the geniohyoid.

_Insertion._--The fibres pass dorsad, diverging in a fan-like manner and forming a flat vertical plate closely applied to the muscle of the opposite side. This plate extends along the caudal three-fourths of the tongue, i.e. as far as it is attached. The cranial fibres arch craniad to the tip of the tongue, the caudal fibres arch caudad to the root of the tongue. Dorsad the muscle is confounded with the muscle of the opposite side.

_Action._--Draws the root of the tongue forward and the tip backward.

=M. hyoglossus= (_h_).--From the body of the hyoid bone to the tongue.

_Origin._--(1) From the ventral surface of the body of the hyoid laterad of the geniohyoid (_g_), and (2) by a second head from the ceratohyal.

_Insertion._--Both heads penetrate into the tongue between the styloglossus (_e_) and the genioglossus (_f_). The fibres intermingle with those of the styloglossus (_e_) and thus help to form the lateral parts of the tongue. They finally end in the integument on the dorsum of the tongue at the sides.

_Action._--Retracts the tongue and depresses it.

=M. styloglossus= (_e_).--From the stylohyal bone to the tongue.

_Origin_ from the mastoid process of the temporal bone, from the stylomandibular ligament (2) (which connects the border of the external auditory meatus with the angular process of the mandible) and from the proximal cartilaginous portion of the cranial cornu of the hyoid bone. The fibres pass mediad, diverging between those of the digastric and hyoglossus (_h_) into the lateral part of the tongue.

_Insertion._--The fibres pass toward the tip of the tongue, where the mass finally ends in a point, the superficial ones gaining insertion into the integument at the sides of the tongue.

_Relations._--Outer surface with the digastric (Fig. 65, _b_) and mylohyoid (Fig. 65, _c_). Inner surface with the pterygoideus internus (Fig. 96, _d_), the cranial cornu of the hyoid, and the tympanic bulla.

_Action._--Retracts the tongue and raises it.

The =intrinsic muscles= of the tongue (those entirely within it) are attached to its integument at both their ends. There are three sets of fibres: a longitudinal, a transverse, and a vertical one. These are seen most readily in cross-sections.

THE SOFT PALATE. VELUM PALATINUM (Fig. 66, page 112).--The soft palate is the free curtain-like structure which forms the caudal part of the roof of the mouth. It is attached to the caudal border of the palatal plates and the ventral border of the perpendicular plates, of the palatine, and to the pterygoid processes and hamuli of the sphenoid, and extends some distance caudad of the hamuli. It thus forms a rather long and narrow curtain separating the caudal part of the nasal cavity from the mouth. Caudad it ends in a free arched border (Fig. 66, 4) which is at about the level of the epiglottis, and may lie against the cranial or the caudal surface of the latter. The narrowed passage bounded by the margin of the velum palatinum dorsad and the tongue ventrad is the =isthmus faucium=. From the sides of the velum a short distance from the caudal border a fold of mucosa passes ventrad to the side of the tongue; a short distance caudad of this a similar fold passes to the floor of the pharynx. These folds form the cranial and caudal =pillars of the fauces=. Between these folds is a shallow pocket, from the bottom of which there arises a prominent projection or swelling which is one of the two =tonsils= (Fig. 95, _d_). Each tonsil is a reddish, lobulated gland, lymphoid in the adult, nearly a centimeter in length, and about one-third as long as broad, with its long axis craniocaudad.

The velum palatinum consists of two layers of mucous membrane, oral and nasal, with intervening muscular and connective tissue. The muscles of the soft palate in the cat are as follows:

=M. tensor veli palatini= (Fig. 66, _d_, _d′_, page 112).

_Origin_ from the ventral surface of the body of the sphenoid between the foramen ovale and the groove for the Eustachian tube. The muscle ends in a flat tendon which passes over the hamular process (3) of the pterygoid bone.

_Insertion_ by spreading out in the soft palate into an aponeurosis which joins the aponeurosis of the opposite muscle and lies between the mucous membrane of the mouth and that of the nose.

_Action._--Stretches the palate.

=M. levator veli palatini= (Fig. 66, _e_, _e′_).--A flat triangular muscle which lies within the tensor.

_Origin_ from the surface of the body of the sphenoid mediad of the groove for the Eustachian tube, from the styliform process of the bulla tympani, and in part from the Eustachian tube. The muscle passes caudad, and its fibres then diverge into the velum palatinum.

_Insertion_ into the velum palatinum, some of the fibres meeting in the middle line.

_Action_ indicated by the name.

A number of other muscles have been described in the soft palate of the cat; they are, however, poorly developed and not easily distinguished. For an account of these, see Stowell, Proceedings of the Am. Soc. of Microscopists, 1889.

2. =The Pharynx.=--At the caudal end of the mouth cavity the passage for the food and that for the air cross; at the cranial end the food-passage (mouth) is ventral, the respiratory passage (nasal cavity) dorsal. Farther caudad the food-passage (œsophagus) is dorsal, while the respiratory passage (larynx and trachea) is ventral. In the region of crossing there is therefore for a certain distance a common passageway for food and air, and this is known as the =pharynx=. It extends from the isthmus faucium, at the free caudal margin of the soft palate, to the beginning of the œsophagus, at the dorsal or caudal margin of the opening of the larynx. The dorsal wall of the pharynx is separated from the base of the skull and the centra of the cervical vertebræ only by intervening muscles (=longus capitis=, =levator scapulæ ventralis=, and =longus colli=, Fig. 72, page 143). Its lateral and ventral walls are supported by the hyoid bone and the cartilages of the larynx.