as they go, and finally empty on the surface of the conjunctiva. Aside from these, the lateral portion of the gland possesses also independent ducts. All the excretory ducts are lined by columnar epithelium and surrounded by a relatively thick connective-tissue wall having inner longitudinal and outer circular fibers. From the lateral portion of the conjunctival culdesac, into which the secretion is brought by the excretory ducts of the lacrimal gland, the secretion passes into the capillary space of the sac, and is then evenly distributed by means of the sulci and papillæ over the conjunctival surface of the lid. In this manner the secretion reaches the mesial angle of the lid, whence it passes through the lacrimal puncta into the lacrimal canals.

The nerve supply of the lacrimal glands is from the sympathetic nervous system. The neuraxes of sympathetic neurones accompany the gland ducts and form plexuses about the alveoli, the terminal branches of which may be traced to the gland cells.

The lacrimal canals are lined by stratified squamous epithelium, and possess a basement membrane as well as a nective-tissue layer containing circularly disposed elastic elements. Externally we find a layer of transversely striated muscle-fibers.

The lacrimal sac is provided with a simple pseudostratified columnar epithelium having two strata of nuclei. In it goblet cells are also found. The nasal duct is lined by a similar epithelium. The connective-tissue wall of the latter and that of the lacrimal sac come in contact with the periosteum; between them is a welldeveloped vascular plexus. Stratified squamous and ciliated epithelium have been described as being present in the nasal duct, as well as mucous glands in both nasal duct and lacrimal sac. (See works of M. Schultze, 72; Schwalbe, 87.)

TECHNIC.

316. The eyes of the larger animals, after having been previously cleaned by removing the muscles and loose connective tissue, are placed in the fixing fluid and cut into two equal parts by means of an equatorial incision. Smaller eyes with thin walls may be fixed whole.

Müller's fluid (T. 27), nitric acid (T. 25), and Flemming's fluid (T. 17) are usually employed as fixing agents. After fixing in one of these fluids, different parts of the eyeball are imbedded in celloidin or celloidin-paraffin and then sectioned.

317. The corneal epithelium is best macerated in 33% alcohol; the membrane of Descemet may be impregnated with silver. In order to bring the fibers of the latter into view, Nuel recommends an injection of 1% to 2% formic acid into the anterior chamber of the eye of a dove or a rabbit, after having drawn off the aqueous humor. The cornea is then cut out, and fixed for from three to five minutes in osmic acid.

318. The substantia propria is examined either by means of sections or by means of teased preparations from a cornea macerated in limewater or potassium permanganate. The sections are stained with picro

carmin (Ranvier). The corneal spaces and canaliculi may be demonstrated in two ways with the aid of silver nitrate; either the fresh cornea of a small animal is stripped of its epithelium, cauterized with a solid stick of silver nitrate, and then examined in water, in which case the corneal spaces and their canaliculi show light upon a dark ground (negative impregnation); or the corneæ of larger animals are treated in the same manner, after which tangential sections are made with a razor, and placed in water for a few days; in this case the corneal spaces and their canaliculi show dark upon a light ground (positive impregnation, Ranvier, 89).

319. By means of Altmann's oil method (T. 112) casts of the corneal spaces and their canaliculi may be made. Treatment by the gold method often brings out not only the nerves, but also the corneal corpuscles and their processes.

320. Ranvier (89) especially recommends a 1% solution of the double chlorid of gold and potassium for the corneal nerves. The cornea of the frog is treated for five minutes with lemon-juice, then for a quarter of an hour with 1% potassium-gold chlorid solution, and, finally, for one or two days with water weakly acidulated with acetic acid (2 drops to 30 c.c. of water), the whole process taking place in the light. Golgi's method may also be used, but the gold method is more certain. 321. The sclera is treated in a similar manner.

322. The pigmentation of the vascular layer interferes with examination, and albinotic animals should therefore be selected; or the pigment may be removed from the previously fixed eyeball with hydrogen peroxid or nascent chlorin. The latter method is applied exactly as in cases where the removal of osmic acid is desired (T. 144).

323. The adult lens is sectioned with difficulty, as it becomes very hard in all fixing fluids. The anterior capsule of the lens may be removed from previously fixed specimens and examined by itself. The lens-fibers are demonstrated by maceration in 3 alcohol (twenty-four hours) or in strong nitric acid. Before immersion the lens-capsule is opened by a

puncture.

324. The retina can rarely be kept unwrinkled in eyes that have been fixed whole. The eyeball should therefore be opened in the fixing fluid and the latter permitted to act internally; or the external tunics are removed, thereby enabling the fixing fluid to act externally.

325. Ranvier recommends subjecting the eyes of smaller animals (mouse, triton) for a quarter or half hour to the action of osmic acid fumes (vid. T. 16), after which the eyes are opened in 3 alcohol with the scissors. At the end of three or four hours the posterior half of the eye is stained for some time in picrocarmin (T. 67), then carried over into 1% osmic acid for twelve hours, washed with water, treated with alcohol, and cut.

In osmic acid preparations the rod-nuclei show dark transverse bands, a condition due to the fact that the end-regions of the nuclei stain more deeply.

The retina is a good object for differential staining, as, for instance, with hematoxylin-eosin, hematoxylin-orange G, etc. The latter combination is particularly successful in staining the rod- and cone-ellipsoids. The examination of tangential sections should not be omitted.

326. With the retina the best results are obtained by means of Golgi's method. Attention must be called to the fact that the supporting structures of the retina are more easily impregnated than the nervous elements, and that the latter can be demonstrated to any extent only in very young

eyes.

327. Ramón y Cajal (94) recommends the following method, modified after Golgi: After the removal of the vitreous humor the posterior half of the eyeball is placed for one or two days in a mixture containing 3% potassium bichromate 20 c.c. and 1% osmic acid 5 or 6 c.c. The pieces are then dried with tissue paper and placed in a 0.75% silver nitrate solution for an equal length of time. Without washing, the pieces are immersed for from twenty-four to thirty-six hours in a mixture containing 3% potassium bichromate 20 c.c., and 1% osmic acid 2 or 3 c.c., and then again carried over into a 0.75% silver nitrate solution for twenty-four hours. In order to prevent precipitation it is advisable to roll up the retina before treating, and to cover it with a thin layer of a thin celloidin solution, which prevents it from again unrolling.

328. The methylene-blue method (T. 312) will also bring out the nervous elements of the retina, although the results are not quite so satisfactory as those obtained by Golgi's method.

IX. THE ORGAN OF HEARING.

THE ear, the organ of hearing, consists of three parts: (1) The external ear, including the pinna or auricle and the external auditory canal; (2) the middle ear, tympanum, or tympanic cavity, containing the small ear bones and separated from the external auditory canal by the tympanic membrane, but communicating with the pharynx by means of the Eustachian tube; (3) the inner ear, or labyrinth, consisting of a bony and a membranous portion, the latter lined by epithelial cells, especially differentiated in certain regions to form a neuro-epithelium, in which the auditory nerves terminate. The first two parts serve for the collection and transmission of the sound-waves; the complicated labyrinth, with its differentiated neuro-epithelium, for the perception of the same. Figure 342 presents in a schematic way the relationships of the parts here mentioned.

A. THE EXTERNAL EAR.

The cartilage of the ear, including that of the external auditory passage, is of the elastic variety, but differs from typical elastic cartilage in that it contains areas entirely free from elastic fibers. The elastic reticulum is, however, never absent near the perichondrium. The skin covering the pinna is thin, and in it are found hairs with relatively large sebaceous glands; sweat-glands are found on the outer surface.

The skin lining the cartilaginous portion of the external auditory canal possesses very few pronounced papillæ, and is characterized by the presence of so-called ceruminous glands, which represent modified and very highly differentiated sweat-glands. Two or three of the latter sometimes become confluent, and then possess only a single excretory duct, which, as a rule, empties into a hair follicle. near the surface of the skin. The corium is somewhat mobile.

The skin lining the osseous portion of the external auditory canal is supplied with neither hair nor glands, and possesses slender papillæ, especially in the neighborhood of the tympanic membrane. The corium is closely attached to the periosteum.

The tympanic membrane consists of a tense and a flaccid portion.

Fig. 342.-Schematic representation of the complete auditory apparatus (Schwalbe).

It forms a part of both the external and the middle ear. From without inward, the following layers may be differentiated: (1) the cutaneous layer; (2) the lamina propria; and (3) the mucous layer.

The epidermis of the cutaneous layer is identical in structure with that of the outer skin, except that the superficial layers of the stratum corneum contain nucleated cells. The corium is very thin, except along the course of the manubrium of the malleus, where it is thickened, forming the so-called cuticular ridge, which possesses papillæ and is supplied with vessels and nerves.

The lamina propria ends peripherally in a thickened ring of fibrocartilaginous tissue, the annulus fibrosus, which unites at the sulcus

tympanicus with the periosteum of the latter. The lamina propria is composed of connective-tissue fibers, in which two layers may be distinguished-externally, the radiate fibers, the stratum radiatum, and internally, the circular fibers, the stratum circulare. The external radiate layer extends from the annulus to the umbo and manubrium, and is interrupted in the flaccid portion of the tympanic membrane by the upper fourth of the manubrium and the short process of the malleus; it gradually thins out toward the center. until it finally disappears in the vicinity of the umbo. The fibers of the inner (circular) layer are circularly disposed. This layer is thickest at the periphery of the tympanic membrane, becoming gradually thinner toward the lower end of the manubrium, where it disappears. Between the two layers of the lamina propria is a small quantity of loose connective tissue. The manubrium of the malleus is inclosed within the tympanic membrane. This is due to the union of the fibers of the radial layer with the outer strata of the manubrial perichondrium, the handle of the malleus being here covered by a thin layer of cartilage. In the posterior upper quadrant of the tympanic membrane the two layers of the lamina propria intermingle, forming irregularly disposed bundles and trabeculæ, the dendritic fibrous structures of Gruber.

The mucous layer of the tympanic membrane consists of simple squamous epithelium separated from the lamina propria by a thin connective-tissue layer containing but few cells. It likewise extends over the handle of the malleus. In the flaccid portion of the tympanic membrane the lamina propria disappears, so that in this region the cutaneous layer and the mucous membrane are in direct contact.

B. THE MIDDLE EAR.

The middle ear, or tympanum, is a small irregular cavity, filled with air, situated in the petrous portion of the temporal bone between the bony wall of the inner ear and the tympanic membrane, and communicates with the pharynx through the Eustachian tube. It contains the small bones of the ear, their ligamentous attachments, and, in part, the muscular apparatus moving them.

The mucous membrane lining the tympanic cavity is folded over the ossicles and ligaments of the tympanum and is joined to that of the tympanic membrane and the Eustachian tube, the line of junction with the former being marked by the presence of papilla-like elevations.

The epithelium of this mucous membrane is a simple pseudostratified ciliated epithelium, having two strata of nuclei. Cilia are, however, lacking on the surface of the auditory ossicles, on their ligaments, and on the promontory of the inner wall, as well as on the tympanic membrane. The mucosa of the mucous membrane is intimately connected with the periosteum, and contains short isolated