44-6 THE EYE. 



the sections are in the stain until the solution steams ; allow the stain to 

 cool ; rinse in distilled water ; wash in a i % aqueous solution of glacial 

 acetic acid for a few seconds or in acid alcohol (six drops of hydrochloric 

 acid; 70% alcohol looc.c.) for a few seconds ; dry sections with filter- 

 paper ; dip sections a few times in absolute alcohol ; differentiate in cre- 

 osote, ten minutes to an hour control now and then under the micro- 

 scope ; wash in several xylols and mount in xylol -balsam. Neuroglia fibers 

 blue, chromatin of neuroglia cell nuclei a purplish blue, protoplasm of 

 neuroglia cells brownish red to bluish red. 



VIII. THE EYE. 



A. GENERAL STRUCTURE. 



THE organ of vision consists of the eyeball, or bulbus oculi, 

 and the entering optic nerve. 



In the eyeball we distinguish three tunics : (i) a dense external 

 coat, the tunica fibrosa or externa, which may be regarded as a 

 continuation of the dura mater, consisting of an anterior transparent 

 structure, called the cornea, and the remaining portion, known as 

 the tunica sclerotica, or, briefly, the sclera ; (2) within the tunica 

 fibrosa a vascular tunic, the tunica vasculosa or media, subdivided 

 into the choroid, ciliary body, and iris ; (3) an inner coat, the tunica 

 interna, which consists of two layers, the inner being the retina ; 

 the outer, the pigment membrane. The latter lines the internal 

 surface of the tunica vasculosa throughout. Within the eyeball 

 are the aqueous humor, the lens, and the vitreous body. The lens is 

 attached to the ciliary body by a special accessory apparatus the 

 zonula ciliaris. These two structures the lens and its fixation 

 apparatus divide the cavity of the eyeball into two principal cham- 

 bers, the one containing the aqueous humor and the other the 

 vitreous. The former is further subdivided by the iris into an 

 anterior and a posterior chamber. During life the latter is only a 

 narrow capillary cleft. 



B. DEVELOPMENT OF THE EYE* 



In man the eyes begin to develop during the fourth week of 

 embryonic life, and at first consist of a pair of ventrolateral diver- 

 ticula, projecting from the anterior brain vesicle. These evaginations 

 gradually push outward toward the ectoderm, and are then known 

 as the primary optic vesicles. The slender commissural segments 

 connecting the vesicles with the developing brain are termed the 

 optic stalks. 



Very soon a process of invagination takes place ; that portion 

 of the vesicular wall nearest the ectoderm is pushed inward, thus 



