204 VISCERAL ANATOMY. 



External Limiting Membrane, a membranous expansion of radial con. 



nective tissue, perforated by foramina (shown in the cut by a line between 



6 and *). 

 Rods and Cones, or Jacob's Membrane? the perceptive layer of the retina, 



composed of rods and cones closely packed together, like palisades, the 



termini, probably, of the optic nerve fibres. 

 [A tenth layer is described by some anatomists, the pigmentary layer of the 



choroid. 8 See ante, p. 200.] 



Name the Essential Elements of the Retinal Structure. 

 PRINCIPAL ELEMENTS, are three, as follows, viz. 



Nerve fibres, which in the molecular layers break up into very delicate 



plexuses. In their course are found 

 Nerve-cells, ordinary bipolar or multipolar, and modified ones, the so-called 



granular layers of the retina. 

 Terminal Organs, the rods and cones of Jacob. 

 SUBORDINATE ELEMENTS, are two, the 



Modified Connective Tissue, of spongy nature. 

 Vessels, blood-vessels and lymphatics. 



THE HUMORS. 



What is the Aqueous Humor ? It is a clear, alkaline, serous fluid, com- 

 posed of water 96.7, albumen o.i, chloride of. sodium and extractive matters 

 3.2, weighing 4 to 5 grains, and filling the anterior and posterior aqueous 

 chambers, which communicate with each other when the pupil is dilated 

 sufficiently to remove the pupillary margin of the iris from the surface of the 

 lens. The 



Anterior Chamber of the Aqueous Humor, has the cornea in front and the 



iris behind. 



Posterior Chamber of the Aqueous Humor, has the iris in front, the lens, 

 zonula and ciliary body behind. [The existence of any such space is 

 denied by many authorities.] 



Describe the Crystalline Lens. It is a bi-convex, elastic, transparent 

 body, enclosed in a capsule, held in place by a suspensory ligament, and situ 

 ated between the posterior aqueous chamber and the chamber of the vitreous 

 humor, immediately behind the pupil. Its posterior surface is the most convex, 

 and rests in the Hyaloid Fossa of the vitreous. Its 



Dimensions and Weight, diameter, about ^ inch, axis about \ inch, 



weight, 4 to 4^ grains, index of refraction, 1.44. 



Composition, water 60, soluble albuminous matter 35, insoluble albuminous 

 matter 2.5, fat and cholesterine 2 per cent. 



