Part 36

The =flexor pollicis brevis= arises on the posterior carpal ligament over the second metacarpal bone, and ends on the volar sesamoid of the first digit.

The =adductor pollicis=, situated externally to the preceding, is the largest of the thumb muscles. It arises between the preceding and the second interosseous muscle, and is inserted into the first phalanx of the first digit.

The =adductor digiti secundi= is situated between the second interosseous muscle and the adductor digiti quinti. It arises on the carpal ligament, and ends on the first phalanx of the second digit.

The =adductor digiti quinti= arises close to the preceding muscle, and passes outward to end on the first phalanx of the fifth digit.

The =flexor digiti quinti= arises on the ligament connecting the accessory carpal to the third and fourth metacarpal bones, crosses the corresponding interosseous muscle, and ends on the fifth digit with the next muscle.

The =abductor digiti quinti= is larger than the two preceding muscles; it arises on the accessory carpal bone, and ends on the outer sesamoid of the fifth digit and on the lateral ligament.

There are four =interossei= which lie on the volar (posterior) surface of the metacarpus. They are well developed and fleshy. They arise on the distal row of the carpus and on the proximal ends of the metacarpals. Each divides distally into two branches, which are inserted by small tendons on the corresponding sesamoid bones, and detach slips to the extensor tendons.

MUSCLES OF THE NECK

The =sterno-cephalicus= is well developed. It arises on the manubrium sterni and ends on the mastoid process, blending with the cleido-cervicalis.

The =scalenus ventralis= (s. primæ costæ) arises on the last four cervical transverse processes, and is inserted into the first rib.

The =scalenus dorsalis= (s. supracostalis) is large. Anteriorly it blends with the preceding muscle, while posteriorly it divides into two parts. The upper part is inserted on the third and fourth ribs, the =lower part= by a long, thin tendon on the eighth rib.

The =longus colli= resembles that of the horse.

The =rectus capitis anterior major= arises on the transverse processes of the second to the sixth cervical vertebræ and ends as in the horse. The =rectus minor= and =lateralis= resemble those of the horse.

The =intertransversales= resemble those of the ox.

The =splenius= is strong and extensive. It arises on the first four or five thoracic spines and the median raphé of the neck, and is inserted into the occipital crest and mastoid process.

The =complexus= is composed of two parts—the biventer cervicis and the complexus major proper. The =biventer cervicis= arises from the transverse processes of the fifth and sixth, and the spines of the second to the fifth (or sixth) thoracic vertebræ, from the ligamentum nuchæ, and the median raphé. It has four tendinous intersections. The =complexus major= arises on the transverse processes of the first three or four thoracic vertebræ and the articular processes of the last five cervical. Both end on a strong common tendon which is inserted into the occipital crest and the depression below it.

The =trachelo-mastoideus= consists of two unequal portions. The large =dorsal part= (M. longissimus capitis) arises on the transverse processes of the first four thoracic and the articular process of the last three or four cervical vertebræ, and ends with the splenius on the mastoid process of the temporal bone. The small =ventral part= (M. longissimus atlantis) arises on the articular processes of the third, fourth, and fifth cervical vertebræ, and ends on the wing of the atlas.

The other muscles present no striking differential features, but it may be noted that distinct =interspinales= are present.

MUSCLES OF THE THORAX

There are twelve pairs of =levatores costarum=.

The =external intercostals= do not occupy the spaces between the costal cartilages.

The =diaphragm= has a small tendinous center. The =œsophageal opening= is between the pillars. The fleshy rim is attached at the costo-chondral junctions from the eighth backward, and along the thirteenth rib nearly its entire length.

MUSCLES OF THE BACK AND LOINS

The =serratus anticus= arises from the median raphé of the neck and the first six or seven thoracic spines, and is inserted into the second to the ninth ribs. It is well developed. The =serratus posticus=—much weaker—arises on the lumbo-dorsal fascia, and is inserted into the last three or four ribs. Thus one or two ribs intervene between the two.

The =transversalis costarum= (Iliocostalis) is well developed, and extends from the ilium to the sixth, fifth, or fourth cervical vertebra.

The =longissimus= resembles that of the other animals, but the spinalis et semispinalis separates clearly at the sixth or seventh thoracic vertebra. It is inserted into the articular and spinous processes of the last six cervical vertebræ. It has no depression in the lumbar region for the gluteus medius.

The =intertransversales= are fleshy, as in the ox.

The =interspinales= are distinct, especially in the lumbar region.

MUSCLES OF THE TAIL

These present the same general arrangement as in the horse. The =sacro-coccygei=, however, arise on the lumbar vertebræ also, and the =coccygeus= on the ischiatic spine. There is found a =sacro-coccygeus accessorius=, which arises on the internal border of the ilium, the edge of the sacrum, and the transverse processes of the first coccygeal vertebræ, and is inserted between the superior and lateral sacro-coccygei. It is homologous with the intertransversales.

MUSCLES OF THE ABDOMEN

The abdominal tunic is practically absent.

The =obliquus abdominis externus= has an extensive fleshy portion. It arises from the last eight or nine ribs and the lumbo-dorsal fascia.

The =obliquus abdominis internus= arises from the external angle of the ilium and the lumbo-dorsal fascia. The fibers have an almost vertical direction, and there is a fleshy attachment to the last rib.

The =rectus abdominis= is attached by a long tendon on the first five or six costal cartilages, and by fleshy fibers on the xiphoid cartilage. It has three to six indistinct tendinous inscriptions.

The =transversus abdominis= presents no special features except that the posterior part of its aponeurosis splits into two layers which include the rectus between them.

MUSCLES OF THE PELVIC LIMB

[Illustration:

FIG. 235.—MUSCLES OF TAIL, ANUS, AND GENITAL ORGANS OF DOG.

_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.

[Illustration:

FIG. 236.—MUSCLES OF PELVIC LIMB OF DOG, INTERNAL VIEW.

_a_, Ilio-psoas; _b_, tendon of psoas minor; _c_, sacro-coccygeus ventralis lateralis; _d_, coccygeus; _e_, pyriformis; _f_, obturator internus; _g_, origin of retractor ani; _h_, _h′_, sartorius; _i_, rectus femoris; _k_, vastus internus; _l_, pectineus; _m_, adductor; _n_, gracilis; _o_, semitendinosus; _p_, semimembranosus; _q_, gastrocnemius, inner head; _q′_, tendon of gastrocnemius; _r_, superficial digital flexor; _r′_, tendon of _r_; _s_, tarsal tendon of biceps femoris; _t_, popliteus; _u_, deep digital flexor, outer head (flexor hallucis longus); _u′_, long digital flexor; _u″_, common tendon of u and _u′_; _v_, tendon of tibialis posterior; _w_, tibialis anterior; _x_, tendon of anterior or long digital extensor; _1_, pelvic surface of ilium; _2_, section of symphysis pelvis; _3_, tuber ischii; _4_, internal condyle of femur; _5_, ligamentum patellæ; _6_, _7_, internal surface of tibia; _8_, tarsus; _8′_, fibular tarsal bone; _9_, metatarsus. (Ellenberger-Baum, Anat. d. Hundes.) ]

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.

[Illustration:

FIG. 237.—MUSCLES OF LEG AND FOOT OF DOG, EXTERNAL VIEW.

_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=[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.

[Illustration:

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.

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.