Part 39

_1_, Ilium; _2_, femur; _3_, tuber ischii; _4_, sacro-sciatic lig.; _5_, sacral region; _6_, tail; _7_, penis; _8_, anus; _9_, rectum; _a_, sacro-coccygeus superior; _b_, sacro-coccygeus accessorius; _c_, coccygeus; _d_, sacro-coccygeus inferior; _e_, retractor ani; _f_, _f′_, sphincter ani externus; _g_, retractor penis; _h_, bulbo-cavernosus; _i_, transversus perinei (?); _k_, _m_, ischio-urethrales; _l_, ischio-cavernosus; _n_, tendon of obturator internus; _o_, gemellus; _p_, urethral muscle. (After Ellenberger, in Leisering’s Atlas.) ]

The =psoas minor= arises on the last thoracic and first four or five lumbar vertebræ, and is inserted into the ilio-pectineal line.

The =psoas major= is short, arising from the last four lumbar vertebræ.

The outer head of the =iliacus= is small, while the inner head is large and fuses with the psoas major.

The =quadratus lumborum= is well developed, and extends laterally beyond the outer edge of the ilio-psoas. It arises from the last four ribs and the lumbar transverse processes, and ends on the pelvic surface of the wing of the ilium.

The =tensor fasciæ latæ= consists of two parts. The anterior part is long and rounded; the posterior is shorter and fan-like.

The =gluteus superficialis= is small. It arises on the sacrum, first coccygeal vertebræ, and sacro-sciatic ligament. It is inserted below and behind the trochanter major of the femur, on the outer branch of the linea aspera.

The =gluteus medius= has no lumbar portion. It is inserted into the trochanter major by a strong tendon.

The =gluteus profundus= is broad and fan-shaped. It arises on the superior ischiatic spine and on the ilium as far forward as the gluteal line, and is inserted into the trochanter major below the medius.

The =pyriformis= is not blended with the gluteus medius. It arises from the border of the sacrum and from the sacro-sciatic ligament, and ends on the trochanter major.

The =biceps femoris= has two heads of origin which soon fuse. The larger head arises from the sacro-sciatic ligament and tuber ischii, the smaller one from the tuber ischii. The aponeurosis of insertion ends on the patella, the patellar ligament, and the tibial crest. There is also a tendinous band, which comes from the deep face of the muscle and terminates at the tarsus, as in the horse.

The =abductor cruris posterior= may be regarded as an accessory head of the biceps femoris. It is a thin, muscular band which arises on the sacro-sciatic ligament, passes downward between the biceps and semimembranosus, and ends by blending with the former.

The =semitendinosus= arises from the tuber ischii only. It ends as in the horse.

The =semimembranosus= is large and arises from the ischium only. It divides into two portions: the anterior portion ends on the tendon of the pectineus, on the femur above the internal condyle, and on the inner (Vesalian) sesamoid bone; the posterior portion ends on the internal condyle of the tibia, the tendon passing under the internal lateral ligament of the stifle joint.

The =rectus femoris= has only one tendon of origin. The single patellar ligament acts as the tendon of insertion of the quadriceps.

The =capsularis= is usually present, but is small and pale.

The =sartorius= consists of two portions. It arises from the external angle and border of the ilium. The anterior portion ends on the patella, the posterior on the internal surface of the tibia, its tendon blending with that of the gracilis.

The =graciles= are not so much fused at their origin as in the other animals.

The =pectineus= is long and slender. It arises from the ilio-pectineal eminence and ends on the internal branch of the linea aspera above the distal end of the femur.

There are two distinct =adductors=. The small anterior one ends on the proximal third of the femur, the posterior one on the distal part of the femur and on the inner face of the stifle joint.

The =quadratus femoris= is short and strong. The other external rotators of the thigh present no special features.

The =peroneus tertius= is represented by a tendinous band which arises on the internal surface of the tibia below the crest. It passes downward on the inner surface of the tibialis anterior, blends with the annular ligament above the tarsus, and is attached to the joint capsule and the proximal end of the third metatarsal bone.

Arloing and Lesbre say: “The third peroneus is a proper extensor of the fifth digit; it is a very feeble, fleshy band, situated behind the peroneus brevis, which it partially covers. It is attached to the upper part of the fibula, and is continued by a long, delicate tendon which passes in the same malleolar groove with the muscle mentioned; it then crosses behind the tendon of the peroneus longus and extends to the phalanges of the outer digit, where it joins one of the branches of the common extensor.”

The =tibialis anterior= is large and superficial. It arises on the external condyle and crest of the tibia, and is inserted into the first metatarsal bone, or into the first tarsal and second metatarsal.

There are four =extensors of the digits=.

1. The =anterior= or =long digital extensor= (M. extensor digitalis longus) is fusiform, and lies largely under the preceding muscle. It arises from the extensor fossa of the femur. The tendon is bound down by two annular ligaments, and divides below the tarsus into four branches, which end on the distal phalanges of the digits (second to fifth).

2. The =lateral digital extensor= (M. extensor digitalis lateralis) is feeble and semipennate. It arises on the fibula below the head. The tendon passes under the lateral ligament of the tarsus and joins the branch of the tendon of the long extensor for the fifth digit.

3. The =extensor hallucis longus= is a very thin muscle which arises from the fibula under the long extensor. Its delicate tendon accompanies that of the tibialis anterior to the first metatarsal bone, or becomes lost in the fascia.

4. The =extensor digitalis brevis= has three divisions. It arises on the fibular tarsal bone and the adjacent ligaments. The three tendons are inserted into the second, third, and fourth digits, blending with the interossei. (Sometimes there is found a tendon to the rudimentary first digit, which may represent the extensor hallucis brevis. There may be a fourth belly for the tendon to the second digit.)

The =peroneus longus= arises on the external condyle of the tibia, the head of the fibula, and the lateral ligament. The long tendon passes down the leg parallel to the fibula, crosses the plantar (posterior) surface of the tarsus transversely, and ends on the first metatarsal bone. A short branch is detached to the external metatarsal bone.

The =peroneus brevis= is semipennate and arises from the distal half or more of the outer face of the tibia and the fibula. Its tendon accompanies that of the lateral extensor over the external malleolus of the tibia, and ends on the proximal end of the fifth metatarsal bone.

The soleus is absent. (It is present and large in the cat.)

The =gastrocnemius= arises on the rough lines above the condyles of the femur. The heads of origin each contain a bone about the size of a pea—the sesamoid of Vesal—which articulates with the corresponding condyle of the femur. The tendon comports itself as in the horse.

The =superficial digital flexor= has a large round belly. It arises in common with the outer head of the gastrocnemius from the external rough line and Vesalian sesamoid bone, and from the aponeurosis of the vastus externus. The tendon winds around that of the gastrocnemius, passes over the tuber calcis (where it is arranged as in the horse), and divides below the tarsus into two branches. Each of these divides into two branches which end as in the fore limb. The outer and inner branches detach slips to the suspensory ligaments of the large digital pad. Muscle-fibers often occur in the tendon in the metatarsal region.

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=[77] (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.

=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.

The =cheeks= (Buccæ) form the sides of the mouth, and are continuous in front with the lips. They are attached above and below to the alveolar borders of the bones of the jaws.

=Structure.=—This comprises: (1) The skin; (2) the muscular and glandular layer; (3) the mucous membrane. The skin offers nothing in particular to notice. The muscular tissue is formed mainly by the buccinator, but also by parts of the panniculus, zygomaticus, dilatator naris lateralis, levator nasolabialis, and depressor labii inferioris. The =buccal glands= (Glandulæ buccales) are arranged in two rows. The upper row (superior buccal or molar glands) is found on the outer surface of the buccinator muscle, near its upper border. The anterior part of the row consists of scattered lobules; the posterior part, which lies under cover of the masseter muscle, is more developed and compact. The lower row (inferior buccal or molar glands), less voluminous than the upper, is situated in the submucous tissue at the lower border of the buccinator muscle. The mucous membrane is reflected above and below upon the gums, and is continuous behind with that of the soft palate. It is reddish in color and frequently shows pigmented areas. Opposite the third upper cheek tooth is the opening of the parotid (Stenson’s) duct, surrounded by a circular fold of the mucous membrane. A linear series of small papillæ above and below indicates the orifices of the small ducts from the buccal glands.

=Blood-vessels and Nerves.=—The blood-supply is derived from the facial and buccinator arteries. The sensory nerves come from the trigeminus and the motor from the facial nerve.

The =gums= (Gingivæ) are composed of a dense fibrous tissue intimately united with the periosteum of the alveolar processes, and blending at the edges of the alveoli with the alveolar periosteum, which fixes the teeth in their cavities. They are covered by a smooth mucous membrane, destitute of glands, and of a low degree of sensibility.

The =hard palate= (Palatum durum) is bounded in front and on the sides by the alveolar arches; behind it is continuous with the soft palate. Its osseous basis is formed by the premaxilla, maxilla, and palate bones. The mucous membrane is smooth, and is attached to the bones by a submucosa which contains in its anterior part an exceedingly rich venous plexus, constituting an erectile tissue. A central raphé (Raphé palati) divides the surface into two equal portions. Each of these presents about eighteen transverse curved ridges (Rugæ palatini) which have their concavity and their free edges directed backward. They are further apart and more prominent anteriorly. There are no glands in the submucosa.

=Vessels and Nerves.=—The blood-supply is derived chiefly from the palatine arteries and the nerves from the trigeminus.

The =soft palate= (Palatum molle) is a musculo-membranous curtain which separates the cavity of the mouth from that of the pharynx. It slopes downward and backward from its junction with the hard palate. The =oral surface= faces downward and forward, and is covered with a mucous membrane continuous with that of the hard palate. It is corrugated and presents numerous small orifices (of gland-ducts) and two sagittal ridges. On each side a short, thick fold passes to join the base of the tongue; this is the =anterior pillar= of the soft palate (Arcus glossopalatinus). The =pharyngeal surface= looks upward and backward and is covered by a mucous membrane continuous with that of the nasal cavity. The =free border= (Arcus palatinus) is concave and thin; it is in contact (except during deglutition) with the epiglottis. It is continuous with a fold of the mucous membrane, which passes on each side along the lateral wall of the pharynx and unites with its fellow over the beginning of the œsophagus; this fold is termed the =posterior pillar= of the soft palate (Arcus pharyngopalatinus). The space between the diverging anterior and posterior pillars (Sinus tonsillaris) is occupied by the faucial =tonsil=. In the horse, however, there is not a compact tonsil, as in man, dog, etc., but a somewhat extended group of mucous glands and masses of lymphoid tissue. These cause elevations of the surface, on which are seen numerous depressions (crypts) in which the gland-ducts open. The soft palate is greatly developed in equidæ, its length being five to six inches (12 to 15 cm.). Its length and contact with the epiglottis account for the fact that in these animals mouth-breathing does not occur under normal conditions, and that in vomiting the ejected matter escapes usually through the nasal cavity.[78]

=Structure.=—The soft palate consists of: (1) The oral mucous membrane, continuous with that of the hard palate; (2) the =palatine glands= (Glandulæ palatinæ), which form a layer about half an inch in thickness; (3) the aponeurotic and muscular layer; (4) the pharyngeal mucous membrane, continuous with that of the nasal cavity.

The =muscles= proper to the soft palate are the azygos uvulæ, the levator palati, and the tensor palati.

The =azygos uvulæ= (M. palatinus) consists of two small muscular bundles which lie together at the median line. It is attached through the medium of the palatine aponeurosis to the palatine arch, and terminates near the free edge of the soft palate. Its action is to shorten and raise the soft palate.

The =levator palati= (M. levator veli palatini) arises from the muscular process of the petrous temporal bone and from the Eustachian tube, and passes forward and downward externally to the latter, to spread out on the pharyngeal surface of the soft palate. It raises the soft palate, thus closing the posterior nares during deglutition.

The =tensor palati= (M. tensor veli palatini) is larger than the levator, and is fusiform and flattened. It arises from the muscular process of the petrous temporal bone and the Eustachian tube, and passes forward external to the levator. Its tendon is then reflected around the hamulus of the pterygoid bone, being held in position by a fibrous band and lubricated by a bursa. It turns inward and expands in the aponeurosis of the soft palate. It tenses the soft palate.

=Vessels and Nerves.=—The blood-supply of the soft palate is derived from the internal and external maxillary arteries. The nerves come from the trigeminus, vagus, and glosso-pharyngeal nerves.