Part 220

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.

FIG. 549.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF DOG, ANTERIOR VIEW (SCHEMATIC).

p, Dorsal proper digital nerves. ]

FIG. 550.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF DOG, POSTERIOR VIEW (SCHEMATIC).

p, Plantar 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.

FIG. 551.—LEFT EYE OF HORSE.

9, Zygomatic arch; 10, supraorbital depression; 12, supraorbital process; 27, facial crest. (After Ellenberger-Baum, Anat. für Künstler.) ]

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.

FIG. 552.—VERTICAL, SECTION OF ANTERIOR PART OF EYE OF HORSE, WITH LIDS HALF CLOSED.

1, Tarsal gland of upper lid; 2, palpebral conjunctiva; 3, fornix conjunctivæ; 4, levator palpebræ superioris; 5, orbicularis oculi; 6, cornea; 7, anterior chamber; 8, iris; 9, 9′, granula iridis; 10, posterior chamber; 11, ciliary process; 12, ciliary muscle; 13, ciliary zone or suspensory lig. of lens; 14, chorioid; 15, sclera; 16, lens; 17, root of tactile hair. (After Bayer, Augenheilkunde.) ]

=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.

FIG. 553.—PIECE OF UPPER EYELID, INNER SURFACE.

1, Cilia; 2, limbus palp. posterior; 3, tarsal glands. (After Ellenberger, in Leisering’s Atlas.) ]

FIG. 554.—CARTILAGE OF THIRD EYELID OF HORSE, CONVEX SURFACE.

1, Gland; 2, fat surrounding deep part of cartilage. (After Ellenberger, in Leisering’s Atlas.) ]

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.