Part 87

The =musculo-cutaneous nerve= passes between the coraco-brachialis and the brachial artery and descends in the arm in front of the artery. At the shoulder joint it gives off branches to the biceps and coraco-brachialis, and in the lower third of the arm is connected with the median nerve by an oblique branch. It terminates near the elbow by dividing into a branch for the brachialis and a small cutaneous nerve which passes down over the inner face of the elbow and, inclining a little forward, descends over the deep fascia of the forearm to the carpus.

The =radial nerve= descends behind the ulnar nerve, gives branches to the extensors of the elbow, dips in between the internal head of the triceps and the accessory head of the anconeus, winds around the arm, and divides between the brachialis and the external head of the triceps into two branches. The =deep branch= (R. profundus) supplies the extensor and supinator muscles on the forearm. The =superficial branch= (R. superficialis) emerges upon the flexor surface of the elbow and divides into two branches which terminate by supplying two =dorsal digital nerves= to each digit, except the fifth, which receives its outer dorsal nerve from the ulnar. The inner branch descends along the inner side of the cephalic vein to the carpus, where it divides into dorsal digital nerves for the first digit and the inner side of the second. The outer branch is much larger. It runs downward on the middle of the front of the forearm and supplies the remaining dorsal digital nerves except that to the outer side of the fifth digit.

The =ulnar nerve= is as large as or larger than the median, with which it is united for some distance. At the distal third of the arm it separates from the median and passes over the internal epicondyle of the humerus. At the proximal part of the forearm it gives off the =dorsal branch= (R. dorsalis), which supplies cutaneous twigs to the dorso-lateral surface of the lower part of the forearm and carpus and terminates as the =external dorsal digital nerve= of the fifth digit. Descending under cover of the flexor carpi ulnaris, the ulnar nerve inclines inward under the tendon of insertion of that muscle and divides into superficial and deep branches. The =superficial branch= (R. superficialis) runs downward along the outer border of the flexor tendons, gives off the =external volar digital nerve of the fifth digit= (N. dig. vol. lat. dig. V), and a branch (N. met. vol. IV) which descends in the space between the fourth and fifth metacarpal bones and unites with the deep branch. The =deep branch= (R. profundus) descends in the carpal canal and divides under the deep flexor tendon into its terminal branches. The smaller of these supply the volar metacarpal muscles. The larger terminals are the three =volar common digital nerves= (Nn. dig. vol. comm. II, III, IV), which descend along the second, third, and fourth intermetacarpal spaces, subdivide, and concur with the volar metacarpal branches of the median nerve in forming the =volar proper digital nerves= (Nn. dig. vol. proprii).

The =median nerve= descends behind the brachial artery, passes over the internal epicondyle of the humerus, then under the pronator teres, and continues in the forearm under cover of the flexor carpi radialis. It gives branches below the elbow to the flexor and pronator muscles, and lower down a palmar branch to the skin on the inner and posterior aspect of the carpus, and terminates between the superficial and deep flexor tendons by dividing into three =volar metacarpal nerves= (N. met. vol. I, II, III). These descend in the first, second, and third intermetacarpal spaces and unite with the volar common digital nerves in forming volar proper digital nerves. The arrangement of the digital nerves is indicated in the annexed schematic figures.

The =lumbo-sacral plexus= is formed from the ventral branches of the last five lumbar and first sacral nerves. The more important special facts in regard to the nerves of the pelvic limb are as follows:

The =saphenous nerve= (Fig. 492) is relatively large, and, in fact, might be considered as the continuation of the femoral nerve. It descends at first in front of the femoral artery, then passes over the lower part of the inner surface of the thigh with the saphenous artery, continues down the leg with the dorsal branch of that vessel, and anastomoses with branches of the superficial peroneal nerve. It supplies cutaneous twigs from the stifle to the metatarsus.

The =peroneal nerve= separates from the tibial above the origin of the gastrocnemius, runs downward and forward across the outer head of that muscle, passes between the deep flexor of the digit and the peroneus longus, and divides into superficial and deep branches. The =superficial peroneal nerve= descends the leg along the peroneus tertius and longus, gives off a =dorsal branch= to the first digit (N. cutaneus dorsalis medialis), and divides at the proximal part of the metatarsus into three =dorsal common digital nerves= (Nn. dig. pedis dors. comm. II, III, IV). These descend with the superficial dorsal metatarsal arteries and concur with branches of the deep peroneal nerve in forming =dorsal proper digital nerves= (Nn. dig. ped. prop. dorsales). The =deep peroneal nerve= passes down the leg with the anterior tibial vessels. It gives twigs to the hock joint and the extensor brevis muscle, and divides into three =dorsal metatarsal nerves= (Nn. met. dors. II, III, IV); these descend with the corresponding arteries along the intermetatarsal spaces and unite with the dorsal common digital nerves in supplying the dorsal proper digital nerves.

The =tibial nerve= divides at the tarsus into two plantar branches. The =internal plantar nerve= descends along the inner border of the superficial flexor tendon and divides near the middle of the metatarsus into two branches. Of these, the internal branch constitutes the =first plantar common digital nerve= (N. dig. comm. plant. I). The outer branch descends on the superficial flexor tendon and divides into three =plantar metatarsal nerves= (Nn. met. plant. II, III, IV), which unite with the plantar common digital nerves. The =external plantar nerve= passes down between the flexor tendons, sends twigs to the muscles on the plantar surface of the metatarsus, and divides into three =plantar common digital nerves= (Nn. dig. comm, plant. II, III, IV). These descend with the deep plantar metatarsal arteries in the second, third, and fourth intermetatarsal spaces, receive the plantar metatarsal nerves, and divide into =plantar proper digital nerves=, which pass down the opposed surfaces of the second to the fifth digits.

The =sympathetic system= presents few special features worthy of mention. The =superior cervical ganglion= lies above the ganglion nodosum close to the cranial base and internal to the origin of the internal carotid and occipital arteries. It forms connections directly or through the carotid plexus with the last seven cranial and first cervical nerves. The =cervical trunk= unites with the vagus so intimately as to form a vago-sympathetic trunk. There may be a small =middle cervical ganglion= in front of the inferior cervical ganglion. The latter is usually distinct from the first thoracic ganglion, which is stellate and lies on the longus colli opposite the second rib and intercostal space.

ÆSTHESIOLOGY

THE SENSE ORGANS AND COMMON INTEGUMENT

The =organs of the senses= (Organa sensuum) receive external stimuli and conduct impulses to the brain which result in sensations of sight, hearing, taste, smell, and touch. They consist essentially of specially differentiated cells, the neuro-epithelium, and a conduction path which is simple in the more generalized sense organs, elaborate in those which are highly specialized—the eye and the ear.

THE SENSE ORGANS AND SKIN OF THE HORSE

THE EYE

The =eye= or =organ of vision= (Organon visus) in the broader sense of the term comprises the eyeball or globe of the eye, the optic nerve, and certain accessory organs associated therewith. The accessory organs (Organa oculi accessoria) are the orbital fasciæ and muscles, the eyelids and conjunctiva, and the lacrimal apparatus. These structures will be considered in the order in which they may be most conveniently examined, taking the horse as a type. The bony walls of the orbit have been described in connection with the skull; the periorbita, a fibrous membrane which incloses the eyeball together with its muscles, vessels, and nerves, may be appropriately included in the account of the fasciæ.

THE EYELIDS AND CONJUNCTIVA

The =eyelids=, upper and lower (Palpebra superior et inferior), are movable folds of integument situated in front of the eyeball. When closed, they cover the entrance to the orbit and the anterior surface of the eyeball. The upper lid is much more extensive and more movable than the lower one, and its free edge is more concave. The interval between the lids is termed the =palpebral fissure= (Rima palpebrarum). When the eye is closed, it is an oblique slit about two inches (ca. 5 cm.) in length; when open, it is biconvex in outline. The ends of the fissure are the =angles= or =canthi=, and are distinguished as =internal= or =nasal=, and =external= or =temporal= (Angulus oculi medialis, lateralis). The external angle is rounded when the eye is open, but the internal angle is narrowed to form a ᑐ-shaped bay or recess, termed the =lacrimal lake= (Lacus lacrimalis). In this there is a rounded, pigmented prominence known as the =lacrimal caruncle= (Caruncula lacrimalis); it is about the size of a small pea and is covered with modified skin, connected with that of the inner commissure, from which project a number of hairs provided with sebaceous glands. The lids unite on either side and form the =commissures=, internal and external (Commissura palpebrarum medialis, lateralis). The =anterior surface= of the lids (Facies anterior palpebrarum) is convex and presents a variable number of long tactile hairs in addition to the ordinary but very short hairs. A considerable number of tactile hairs are scattered over the lower part of the lower lid, but on the upper lid they are very scanty. The infrapalpebral depression (Sulcus infrapalpebralis) indicates somewhat indistinctly the limit of the lower lid. The upper lid is marked by two furrows when raised. The =posterior surface= (Facies posterior palpebrarum) is adapted to the free surface of the eyeball and is covered by the palpebral conjunctiva. The free border of the lid is smooth and usually black. It has a well defined posterior margin (Limbus palpebralis posterior), along which the ducts of the tarsal glands open. The anterior margin (Limbus palpebralis anterior) bears stiff hairs termed the cilia or eyelashes. On the upper lid the cilia are long and numerous except at its inner third, where they are very small or absent. On the lower lid the cilia are often scarcely distinguishable from the ordinary hairs; in other cases they may be clearly seen except near the external canthus, and are much finer and shorter than those of the upper lid. The edge of each lid is pierced near the internal angle by a minute, slit-like opening, the =punctum lacrimalis=, which is the entrance to the lacrimal duct or canaliculus.

=Structure of the Eyelids.=—The skin of the eyelids is thin and freely movable, except near the free edge, where it is more firmly attached. The underlying subcutaneous tissue is destitute of fat. The =muscular layer= consists chiefly of the elliptical bundles of the orbicularis oculi, with which are associated fibers of the corrugator supercilii in the upper lid and the malaris in the lower lid. At the inner side there is a fibrous band, the =palpebral ligament=, which is attached to the lacrimal tubercle and furnishes origin to some fibers of the orbicularis. At the inner commissure a bundle detached from the orbicularis passes inward behind the lacrimal sac, and is known as the pars lacrimalis or Horner’s muscle. At the outer side an indistinct palpebral raphé occurs where fibers of the orbicularis decussate. The =fibrous layer= is thicker and denser along the free edge of the lid, forming here the =tarsus=. The tarsus furnishes insertion to a layer of unstriped muscle known as the tarsal muscle (of Müller). The =tarsal glands= (Glandulæ tarsales) are partly embedded in the deep face of the tarsus, and are visible when the lid is everted if the conjunctiva is not too strongly pigmented. They are arranged in a linear series, close together, and with their long axes perpendicular to the free edge of each lid. In the upper lid they number forty-five to fifty; in the lower, thirty to thirty-five. Each consists of a tubular duct beset with numerous alveoli, in which a fatty substance, the =palpebral sebum=, is secreted. The =palpebral conjunctiva= lines the posterior surface of the eyelids.

The =conjunctiva= is the mucous membrane which lines the lids as =palpebral conjunctiva= (C. palpebrarum) and is reflected upon the anterior part of the eyeball as =bulbar conjunctiva= (C. bulbi); the line of reflection is termed the =fornix conjunctivæ=. The palpebral part is closely adherent to the tarsus, but is loosely attached further back. It is papillated and is covered with stratified cylindrical epithelium in which many goblet cells are present. In the fornix and its vicinity there are tubular glands (of Henle). Near the inner angle there are numerous lymph nodules. The conjunctiva of the outer part of the upper lid is pierced near the fornix by the orifices of the excretory ducts of the lacrimal gland. The bulbar conjunctiva is loosely attached to the anterior part of the sclera and is pigmented in the vicinity of the corneo-scleral junction. On the cornea it is represented by a stratified epithelium.

When the lids are in apposition, the conjunctiva (including the epithelium of the cornea) forms a complete =conjunctival sac=, as the capillary space between the lids and the eyeball is termed.

The =third eyelid= (Palpebra tertia) is situated at the inner angle of the eye (Figs. 551, 556). It consists of a semilunar fold of the conjunctiva, known as the =membrana nictitans=, which covers and partly incloses a curved plate of hyaline cartilage. Its marginal part is thin and usually more or less pigmented. The cartilage has an elongated quadrilateral outline. The part of it which lies in the membrana is wide and thin. The deep part is narrower and thicker and is embedded in fat at the inner side of the eyeball. Numerous minute lymph nodules occur in the membrana nictitans, and the deep part of the cartilage is surrounded by a gland which resembles the lacrimal gland in structure (Glandula superficialis palpebæ tertiæ).

Ordinarily the third eyelid extends very little over the inner end of the cornea, but when the eyeball is strongly retracted (as in tetanus) the membrana is protruded over it so as to measure about an inch (ca. 2 to 3 cm.) in its middle. This effect results from the pressure of the eyeball and its muscles on the fat which surrounds the deep part of the cartilage.

=Vessels and Nerves.=—The =arteries= which chiefly supply the eyelids and conjunctiva are branches of the ophthalmic and facial arteries, and the blood is drained away by corresponding veins. The =sensory nerves= are branches of the ophthalmic and maxillary divisions of the trigeminus. The =motor nerves= to the orbicularis oculi, corrugator supercilii, and malaris come from the facial nerve, the levator palpebræ superioris is innervated by the oculomotor, and the unstriped muscle of the lids by the sympathetic.

THE LACRIMAL APPARATUS

The =lacrimal apparatus= (Apparatus lacrimalis) comprises: (1) the lacrimal gland, which secretes the clear lacrimal fluid; (2) the excretory ducts of the gland; (3) the two lacrimal ducts or canaliculi, lacrimal sac, and naso-lacrimal duct, which receive the fluid and convey it to the nostril.

The =lacrimal gland= (Glandula lacrimalis) is situated between the supraorbital process and the dorso-lateral surface of the eyeball (Fig. 438). It is flattened, oval in outline, and measures about two inches (ca. 5 cm.) transversely and an inch or more (2.5 to 3 cm.) in the sagittal direction. Its upper face is convex and is related to the concave lower surface of the supraorbital process. The lower face is concave in adaptation to the eyeball, from which it is separated by the periorbita. The =excretory ducts= (Ductuli excretorii) are very small and are twelve to sixteen in number; they open into the outer part of the conjunctival sac along a line a little in front of the fornix conjunctivæ superior. In appearance and structure the gland resembles the parotid. It receives its blood-supply chiefly from the lacrimal artery. The sensory nerve is the lacrimal, and the secretory fibers are derived from the sympathetic.

The =puncta lacrimalia= are the entrances to the two lacrimal ducts. Each is a fine slit-like opening (about 2 mm. long), situated close behind the free edge of the lid and about a third of an inch (ca. 8 mm.) from the internal canthus. The =lacrimal ducts= (Ductus lacrimales), upper and lower, begin at the puncta and converge at the inner commissure to open into the =lacrimal sac=. The latter (Saccus lacrimalis) may be regarded as the dilated origin of the naso-lacrimal duct. It occupies the funnel-like origin of the bony lacrimal canal, and leads to the =naso-lacrimal duct= (Ductus naso-lacrimalis), which passes forward and a little downward along the outer wall of the frontal sinus and the nasal cavity and opens near the lower commissure of the nostril. Its length is about ten to twelve inches (ca. 25 to 30 cm.). In the first part of its course it is inclosed in the osseous lacrimal canal; further forward it lies in the lacrimal groove of the maxilla, covered at first by a plate of cartilage and then by the mucous membrane of the middle meatus. The terminal part lies in the inferior turbinal fold and opens on the skin of the floor of the nostril near the transition to mucous membrane. Accessory openings may occur a little further back.

The first part of the duct, about 6 to 7 mm. in diameter, extends in a gentle curve, convex dorsally, from the internal commissure toward a point just above the level of the infraorbital foramen. The second part (isthmus) is narrower (ca. 3 to 4 mm.); it extends forward and a little ventrally about to a transverse plane through the first cheek tooth and lies in the groove above the inferior turbinal crest. Beyond this the duct inclines upward and widens very considerably, crosses the nasal process of the premaxilla obliquely, and contracts at its termination. The mucous membrane may present valvular folds, the most distinct of which is situated at the origin.

THE PERIORBITA

The =periorbita= or ocular sheath is a conical fibrous membrane which incloses the eyeball with its muscles, vessels, nerves, etc. Its apex is attached around the optic and lacerated foramina, and its base is in part attached to the bony rim of the orbit, in part continuous with the fibrous layer of the lids. Its inner part, which is in contact with the orbital wall, is thin; incorporated with it beneath the root of the supraorbital process is the bar of cartilage around which the superior oblique muscle is reflected. The outer part is thicker, and is strengthened by an elastic band which is attached to the pterygoid crest and furnishes origin to the thin unstriped orbital muscle. A quantity of fat (Corpus adiposum extraorbitale) lies about the periorbita, and within it is the intraorbital adipose tissue (Corpus adiposum intraorbitale) which fills the interstices between the eyeball, muscles, etc.

THE ORBITAL FASCIÆ AND OCULAR MUSCLES (Figs. 438, 439, 555)

The straight muscles of the eyeball and the oblique muscles in part are inclosed in fibrous sheaths (Fasciæ musculares), formed by superficial and deep layers of fascia, which are united by intermuscular septa in the interstices between the muscles. The =superficial fascia= is thin; it blends in front with the fibrous layer of the eyelids and is attached behind around the optic foramen. The =deep fascia= consists anteriorly of two layers, one of which is continuous with the fibrous tissue of the lids, while the other is attached at the corneo-scleral junction.

The posterior part of the eyeball is covered by the =bulbar fascia= or capsule of Tenon (Fascia bulbi), so that between them a lymph space (Spatium interfasciale) is inclosed which communicates with the subdural space along the course of the optic nerve.

The =levator palpebræ superioris= muscle[217] is a thin band about half an inch in width which lies above the rectus superior. It is narrow at its origin above and behind the ethmoidal foramen and ends by an expanded tendon in the upper lid. Its action is to raise the upper lid.

The =recti oculi= (Mm. recti bulbi) are four in number and are designated according to their positions as =superior=, =inferior=, =internal=, and =external=. They are all band-like, arise close together around the optic foramen, and diverge as they pass forward to the eyeball. On reaching the latter they end in thin tendons which are inserted into the sclera in front of the equator of the eyeball.

The =retractor oculi= (M. retractor bulbi) surrounds the optic nerve, and is incompletely divided into four parts which alternate with the recti. They arise around the optic nerve and are inserted into the sclera behind the recti.