302 ANIMAL PHYSIOLOGY. [LESSON 94. 



brane, or conjunctiva : such is the brilliancy of its whiteness at this 

 part, that it is commonly called "the white of the eye." The con- 

 junctiva (human) is a tissue of great vascularity, and a figure of it, 



copied from a preparation, is given (Fig. 

 428). The portion of the preparation 

 selected for illustration, is where the 

 membrane approaches the margin of the 

 upper eye-lid ; in this situation (seen at 

 the lower part of the figure) the capil- 

 laries are very minute, and densely ag- 

 gregated the general surface of the 



Conjunctiva injected, human. membrane ( of both Ms) ig CQvered by 



arteries, veins, and loosely scattered capillaries, such as form the 

 greater part of the figure. The Conjunctiva still lines the upper and 

 lower eye-lids, upon which it is displayed ; and so dense and minute 

 is the arrangement of the capillary plexuses along the line of the 

 lids, that, to unassisted vision, they appear to consist of simple lines 

 of intense redness. 



1369. The choroid is a vascular membrane of a rich chocolate 

 brcwn color upon its external surface, and of a deep black color 

 within. Externally it is connected to the sclerotic coat, internally 

 to the retina. It is pierced posteriorly for the passage of the optic 

 nerve, and anteriorly it is connected with the iris, ciliary processes, 

 and junction of the cornea and sclerotic, by a dense white structure, 

 the ciliary ligament, which surrounds the circumference of the iris 

 like a ring. 



1370. This membrane is composed of three layers : 1. An ex- 

 ternal or venous layer, which consists of veins arranged in a peculiar 

 manner, called vence vorticosce. 2. Middle, or arterial layer, is 

 formed by the ramifications of minute arteries, and secretes upon its 

 surface the pigmentum nigrum. 3. The internal layer presents a 

 beautiful appearance under the microscope ; it is composed of several 

 laminae of hexagonal cells, which contain the granules of pigmentum 

 nigrum (black paint). 



1371. In animals the pigmentum nigrum is replaced by a layer 

 of considerable extent, and of metallic brilliancy, called the tapetum. 



1372. The iris (a rainbow) is so called from its variety of color 

 in different individuals ; it forms a curtain or septum (partition) be- 

 tween the anterior and posterior chambers of the eye, and is pierced 

 in its centre by. a circular opening, called the pupil. 



1373. The ciliary processes consist of a number of highly vascu- 



