QUESTIONS FOR CHAPTER XIII.

279. What does the cutaneous system embrace?

280. Name the two portions of the skin.

281. Describe the stratum Malpighii.

282. What are prickle cells?

283. Describe the stratum granulosum.

284. Describe the stratum lucidum; stratum corneum.

285 Describe the corium.

286. What are the papillæ ?

287. Describe the sebaceous gland.

288. Describe the sweat-gland.

289. Name the divisions of the hair.

290. Describe the shaft. Describe each layer separately.

291. What layers compose the hair follicles?

separately.

292. What are the arrectores pilorum?

293. Name the three portions of the nails.

Describe each

294. Describe the nail groove, the nail bed, and the matrix.

CHAPTER XIV.

THE EYE.

THE eye consists of the anterior and the posterior chambers, which are separated by the iris and lens.

The anterior chamber is limited in front and at the sides by the transparent cornea; posteriorly by the iris and lens.

The posterior chamber is surrounded by the retina, the choroid, and the sclera.

The retina covers the posterior portion of the inner surface of the eye and extends nearly to the ciliary body, where it terminates in a ragged edge, called the ora serrata. In the center of the retina, posteriorly, is an elevated spot, the macula lutea, in the center of which is a slight depression, called fovea centralis.

The choroid surrounds the retina externally, is pierced centrally by the optic nerve in its passage to the retina, and terminates in front in the ciliary process and iris.

The sclera is continuous posteriorly with the sheath of the optic nerve, passes forward, and joins the cornea near the latitude of the iris. Just before its junction with the cornea it is joined by a reflection of the conjunctiva, the epithelium of which becomes modified and continues over the remaining anterior portion of the sclera and the entire cornea.

The lens is a biconvex, transparent body, situated immediately behind and in contact with the iris, and is held in position by the suspensory ligaments which are attached to the ciliary body.

The anterior chamber is filled with a clear fluid, the aqueous humor.

The posterior chamber contains a thin, transparent, jelly-like substance, called the vitreous humor.

Fig. 57.-Diagram of eye: 4, Anterior chamber occupied by aqueous humor B, posterior chamber occupied by vitreous; C, crystalline lens; a, conjunctiva; b, cornea; c, retina; d, choroid coat; e, sclerotic coat; f, rectus internus muscle; g, iris; h, canal of Schlemm; i, corneoscleral junction; j, optic nerve; k, central artery; 1, fovea centralis.

The cornea contains five layers, which, named from without inward, are the anterior epithelium, the anterior limiting membrane, the substantia propria, the posterior limiting membrane, and the posterior endothelium.

The anterior epithelium is a stratified epithelium, the innermost cells of which are cuboid, becoming flatter as they progress outward until the outer layers are squamous.

The anterior limiting membrane, anterior elastic lamina, or membrane of Bowman, is a moderately thick basement membrane, consisting of specialized fibrous tissue.

The substantia propria constitutes the main bulk of

-a

-d
-e

Fig. 58.-Cornea: a, External epithelium; b, anterior elastic lamina; c, substantia propria; d, posterior limiting membrane; e, posterior epithelium.

the cornea, and is composed of obliquely arranged bundles of fibrous tissue, which are bound together by clear cement substance (interfibrillar cement substance) and a few white fibers, which are called the fibræ

arcuatæ.

Between the fiber bundles and in the cement substance lie large numbers of irregular stellate lymphspaces (the lacunae, or corneal spaces), which are joined by a network of small canaliculi. In these spaces,

and following their general outline, lie the corneal corpuscles, which unite by their stellate processes.

The posterior limiting membrane, posterior elastic lamina, or Descemet's membrane, is a thin, clear membrane, resembling the anterior limiting membrane.

The posterior endothelium, or Descemet's endothelium, consists of a single layer of flat endothelial cells, resting directly on the posterior elastic lamina.

After birth, the cornea contains no blood-vessels excepting in a narrow zone (the limbus), just at its periphery.

Lymphatic spaces occur in the lacunæ and canaliculi, and also around the large nerves.

The nerves are branches of the ciliary nerves, enter the substantia propria as medullated fibers, which at once lose their medullary sheath and form a network beneath the anterior epithelium, called the subepithelial plexus, which send still finer branches between the epithelial cells, called the intra-epithelial plexus. A few branches are also sent into the posterior portion of the substantia propria.

THE RETINA.

The retina may be divided structurally into three parts the retina proper, the macula lutea with the fovea centralis, and the ora serrata.

The retina proper contains ten layers. Named from within (in contact with the vitreous humor) outward (to the choroid coat), they are

1. Internal limiting membrane. 2. Layer of nerve-fibers.

3. Layer of ganglion cells.

4. The inner molecular, granular, or reticular layer. 5. Inner nuclear layer.

6. Outer molecular, granular, or reticular layer.