Part 55

=M. scaphocuneiformis.=--A small muscle lying on the sole of the foot in the depression between the lateral cuneiform, medial cuneiform, and the lateral tubercle of the scaphoid. It is hidden by the overlying ligaments.

Origin, the lateral tubercle of the scaphoid bone. It passes distad and laterad.

Insertion, the lateral surface of the medial cuneiform.

Action.--Rotates the medial cuneiform on the scaphoid and would thus act as an opponens of the great toe if the great toe were present.

THE VISCERA.

I. THE BODY CAVITY.

The greater part of the viscera are situated in the body cavity or cœlom. This is divided by the diaphragm into two parts, the =thoracic cavity= and the =abdominal cavity=. Each is lined by a serous membrane, in which the part covering the outer wall of the cavity is distinguished as the =parietal layer= from the part covering the viscera, which is known as the =visceral layer=.

The thoracic cavity is bounded by the thoracic vertebræ, the ribs, the sternum, and the diaphragm. The cranial opening of the cavity is filled by the trachea and œsophagus as they enter from the neck region. The thoracic cavity is lined by two thin layers of tissue, the outer one of which is the =fascia endothoracica=, while the inner is the =pleura=. The fascia endothoracica is a sheet of connective tissue which lines the entire inner surface of the thoracic cavity, descending from the dorsal median line to the heart, and passing into the fibrous layer of the pericardium. The pleura is a thin membrane covering the fascia endothoracica and corresponding to the peritoneum of the abdominal cavity. It forms two sacs, the =pleuræ=, lining respectively the right and left halves of the thoracic cavity. Each of these two sacs is closed, the viscera being suspended within them by folds of the membrane, so that the cavity is everywhere separated from the viscera by a sheet of the pleura. That portion of the pleura which lines the thoracic wall is known as the parietal layer; it may be divided into that covering the ribs (=costal= pleura), and that covering the diaphragm. That portion which covers the viscera is the visceral layer, or, since it covers chiefly the lungs, it may be distinguished as the =pulmonary= pleura. The medial walls of the two pleural sacs come in contact in the median plane, forming a median vertical partition passing lengthwise of the thoracic cavity. This partition is known as the =mediastinal septum=. The space between the two layers which make up the mediastinal septum is known as the mediastinum, or mediastinal cavity; it contains numerous organs of the thorax. Three parts are usually distinguished in this cavity: a =ventral= mediastinal cavity, containing chiefly blood-vessels and the thymus gland; a =middle= mediastinal cavity, enclosing the heart and the anterior and posterior venæ cavæ; and a =dorsal= mediastinal cavity, containing the trachea, the œsophagus, and the aorta.

The abdominal cavity lies caudad of the diaphragm; in it are sometimes distinguished the =abdominal cavity proper=, extending as far caudad as the cranial edge of the pubis, and the =pelvic= cavity, lying caudad of this, in the region surrounded by the innominate bones and the sacrum. The two cavities are not distinctly marked off, so that it is convenient to consider the abdominal cavity as undivided. Both parts are lined by the =peritoneum=.

The =peritoneum= is a thin transparent sheet of connective tissue supporting on its surface a layer of flattened epithelial cells, the peritoneal epithelium. It forms a sac which lines the entire abdominal cavity. This sac is closed in the male; in the female, however, it communicates with the exterior through the uterine (or Fallopian) tubes and uteri. All the organs of the abdominal cavity are outside the sac. In the course of their development these organs have encroached on the peritoneal sac. Each has grown against the outer wall of the sac to a greater or less extent and has forced a part of this wall ahead of it into the cavity. In some cases the encroachment has gone so far that the organ in question lies apparently within the peritoneal cavity, suspended from the wall of the sac by a fold of that wall. The wall may thus be divided into three portions. One of these, the =parietal= layer, lines the wall of the body cavity. The second (the =mesentery= in case of the alimentary canal, or a =ligament= in the case of another organ) suspends the organ from the body wall. Between the layers of each mesentery or ligament blood-vessels may pass to the organs. The third portion or =visceral= layer covers the organ in question, forming its =serous= covering.

The reflections of the peritoneum to form the mesenteries and ligaments may be thus described:

Caudally the peritoneum covers the whole surface of the bladder and is reflected from its ventral wall to the linea alba as the =suspensory ligament= of the bladder. Farther craniad the peritoneum suspends the rectum and colon from the middorsal line, forming the =mesorectum= and =mesocolon=. The mesocolon continues craniad to the level of the caudal end of the right kidney and is broadest at its cranial end. At this end the mesocolon passes into the =mesentery= proper, which suspends the small intestine and is very broad and much folded. Its dorsal border is attached to the median line opposite the caudal end of the kidneys and is very short compared to its very long ventral or intestinal border. Toward the cranial end the mesentery of the jejunum passes gradually into the very much shorter duodenal mesentery. This is drawn out at the caudal end of the duodenum into a fold, the =duodenorenal= ligament which attaches the duodenum to the kidney.

The =mesogastrium= or peritoneal fold for the stomach passes from that part of the median dorsal line lying between the kidneys and the diaphragm, to the greater curvature of the stomach. It does not pass directly to the stomach, but passes first ventrad of the small intestine as far as the pelvis. Thence it turns craniad to reach the greater curvature of the stomach. The fold thus formed is called the =great omentum=. It forms the dorsal and ventral walls of a sac, the =omental sac=, the cavity of which is called the =lesser peritoneal cavity=. The descending limb of the fold forms the dorsal wall of the sac, and its ascending limb forms the ventral wall. Each of these walls is double like a mesentery, so that the great omentum consists of four sheets of peritoneum. Between the two sheets forming the descending limb lies the left half of the pancreas, which passes thence to the right into the duodenal mesentery. A transverse fold passes from the descending limb of the omentum along the cranial border of the pancreas to the duodenal mesentery. Farther to the right the descending limb of the omentum, which is here shorter, encloses the spleen and holds it in position parallel to the greater curvature of the stomach and about one centimeter from it. This part of the great omentum is sometimes called the =gastrosplenic= or =gastrolienal= omentum.

The great omental sac communicates with the peritoneal sac by an opening, the =foramen epiploicum= or =foramen of Winslow=. This opening lies caudad and dextrad of the caudate lobe of the liver. This lobe itself lies within the sac. Along the ventral border of the foramen epiploicum pass the common bile-duct from the liver and the portal vein to the liver.

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.