Part 115

a, Quadriceps femoris; b, gastrocnemius, external head; c, superficial digital flexor; d, deep digital flexor; e, peroneus longus; f, tendon of lateral extensor; g, peroneus brevis; h, long or anterior digital extensor; h′, h″, h‴, tendons of preceding; i, tibialis anterior; k, extensor brevis; l, slips from interossei (m); 1, external condyle of femur; 2, patella; 3, tibia; 4, tuber calcis; 5, 6, annular ligaments. (After Ellenberger, in Leisering’s Atlas.) ]

The =deep digital flexor= has two heads. The large outer head, the =flexor hallucis longus=, arises from the tibia and fibula, filling the interosseous space. The small inner head, =flexor digitalis pedis longus=, also arises from the tibia and fibula, its tendon joining that of the large head below the tarsus. The common tendon detaches a branch to the large digital pad and terminates as in the fore limb.

The =tibialis posterior= is a small but distinct muscle, which arises on the proximal part of the fibula. The thin tendon accompanies that of the flexor longus and ends on the internal lateral ligament of the tarsus.

The tendon of the =popliteus= contains a small sesamoid bone.

The =adductors of the second and fifth digits=, the =lumbricales=, and the =interossei= are arranged as in the thoracic limb.

The =quadratus plantæ= arises on the outer surface of the distal end of the fibular tarsal bone and on the lateral tarsal ligament, passes downward and inward, and terminates on a thin tendon which fuses with that of the deep flexor.

The =abductor digiti quinti= is a very small muscle which consists of two parts. One of these is a tendinous slip which extends from the posterior surface of the proximal part of the fibular tarsal bone to the head of the fifth metatarsal bone; the other part arises from the inner surface of the fibular tarsal bone (or from the tendinous part) and ends on the first phalanx of the fifth digit.

In case the skeleton of the first digit is well developed, there are found three muscles homologous with those of the same digit in the fore limb. These are the =abductor hallucis=, =adductor hallucis=, and =flexor hallucis brevis=.

THE DIGESTIVE SYSTEM

This system consists of the organs directly concerned in the reception and digestion of the food, its passage through the body, and the expulsion of the unabsorbed portion. These organs are conveniently grouped under two heads, viz.: (1) the =alimentary canal=; (2) the =accessory organs=.

The =alimentary canal= (Tractus alimentarius) is a tube, about 100 feet (ca. 30 m.) in length in the horse, and extends from the lips to the anus. It has a complete lining of mucous membrane, external to which is an almost continuous muscular coat. The abdominal portion of the tube is largely covered with a serous membrane—the visceral peritoneum. The canal consists of the following consecutive segments:

1. Mouth.

2. Pharynx.

3. Œsophagus.

4. Stomach.

5. Small intestine.

6. Large intestine.

The =accessory organs= are the teeth, tongue, salivary glands, liver, and pancreas.

DIGESTIVE SYSTEM OF THE HORSE

THE MOUTH

The =mouth= (Cavum oris) is the first part of the alimentary canal. In the horse it is a long cylindrical cavity, and when closed, it is almost entirely filled up by the contained structures. The entrance to it (Rima oris) is closed by the lips. Laterally it is bounded by the cheeks; above, by the hard palate; below, by the body of the mandible and the mylo-hyoid muscles; behind, by the soft palate.

The =cavity= of the mouth is subdivided into two parts by the teeth and alveolar processes. The space external to these and inclosed externally by the lips and cheeks is termed the vestibule of the mouth (Vestibulum oris). In the resting state of the parts the walls of this cavity are in contact, and the space is practically obliterated. Its existence becomes very evident in facial paralysis, when the food tends to collect in it laterally, pouching out the cheeks. The space within the teeth and alveolar processes is termed the mouth cavity proper (Cavum oris proprium). When the teeth are in contact, it communicates with the vestibule only by the interdental spaces and the intervals behind the last molar teeth. Posteriorly it communicates with the pharynx through the isthmus of the fauces.

The =mucous membrane= lining the mouth (Tunica mucosa oris) is continuous at the margin of the lips with the common integument, and behind with the mucous lining of the pharynx. During life it is chiefly of a pink color.

The =lips= (Labia oris) are two musculo-membranous folds which surround the orifice of the mouth. Their angles of union (Anguli oris s. commissuræ labiorum) are situated near the first cheek tooth and are rounded. Each lip presents two surfaces and two borders. The outer surface is covered by the skin, which presents long tactile hairs in addition to the ordinary fine hair. The upper lip shows a shallow median furrow (Philtrum), the lower a rounded prominence, the chin (Mentum). The inner or oral surface is covered with mucous membrane which is commonly more or less pigmented. The small papillæ on this surface show on their summits the openings of the ducts of the labial glands. Small folds of mucous membrane which pass from the lip to the gum represent the frænula labii (superioris, inferioris). The free border of the lip is dense and presents short, very stiff hairs. The attached border is continuous with the surrounding structures, and is adherent to the alveolar borders of the bones of the jaws.

FIG. 238.—SAGITTAL SECTION OF HEAD OF HORSE. THE SECTION IS ABOUT 1 CM. TO THE LEFT OF THE MEDIAN PLANE.

1, Hard palate; 2, soft palate; 3, posterior pillar of soft palate; 4, Eustachian opening; 5, great cornu of hyoid bone; 6, stylo-pharyngeus muscle; 9, glosso-pharyngeal nerve; 10, hypoglossal nerve; 11, perpendicular plate of ethmoid; 12, presphenoid; 13, sphenoidal sinus; 14, occipital bone; 15, median surface of cerebral hemisphere; 16, corpus callosum; 17, septum lucidum; 18, fornix; 19, thalamus opticus; 20, corpora quadrigemina; 21, pons; 22, medulla oblongata; 23, spinal cord; 24, cerebellum; 25, pituitary body; 26, hyo-epiglottic muscle; 27, superior longitudinal muscle of tongue; 28, omo-hyoideus; 29, hyo-glossus; 30, hyoideus transversus; 31, thyroid cartilage (body); S. f., frontal sinus; L. n., lig. nuchæ; C I, atlas; C II, axis; R. c. m., rectus cap. ant. major and minor; E p., epiglottis; A r., arytenoid cartilage; P m., premaxilla; M., mandible. The leader line for the guttural pouch points to the external carotid artery. ]

=Structure.=—The lips are covered externally by the skin, and are lined by mucous membrane; between these are muscular tissue and glands. The skin lies directly on the muscles, many fibers of which are inserted into the former. The muscles have been described (page 214). The =labial glands= (Glandulæ labiales) form a compact mass near the angles; they are numerous in the upper lip, fewer in the lower. The mucous membrane is often pigmented, and is reflected upon the bones of the jaws to form the gums.

=Blood-vessels and Nerves.=—The arteries are derived from the superior and inferior labial and palato-labial arteries. The sensory nerves come from the trigeminus, and the motor from the facial nerve.