352 HUMAN ANATOMY. 



The cones are flask-shaped, with their pointed extremities toward 

 the choroid. They also consist of two portions an outer striated and 

 an inner granular. 



10. The pigmentary layer was formerly described as a layer of the 

 choroid. It has received the name of tapctum niyrum, consisting of a 

 layer of pigmented hexagonal epithelial cells. 



In many of the lower animals, this layer on the posterior surface 

 is destitute of pigment, and is called from its iridescent lustre the 

 tapetum lucidum. 



The radiating fibres, or fibres of Mtiller, consist of connective-tissue 

 fibres, connecting all the layers together, and forming the membrana 

 limitans interna and the membrana limitans externa. 



The structure of the retina at the macula lutca differs in the 

 following manner: the cones only of Jacob's membrane are 

 present; the vesicular layer consists of several layers, and in the 

 outer nuclear layer only the cone-fibres are present. 



At the fovea centralis only the cones of Jacob's membrane,, 

 the outer nuclear layer, and the internal granular layer are 

 present. 



The arteria centralis retina supplies only as far as the inner 

 nuclear layer. 



THE HUMORS OF THE EYE. 



THE AQUEOUS HUMOR is a transparent, alkaline, serous fluid, 

 small in quantity, weighing four to five grains, and filling the 

 anterior and posterior chambers of the eye. It is composed of 

 water, 96.7; extractive matters, principally chloride of sodium 

 and albumen, 0.1. 



The anterior chamber is the space (about 2.7 millimetres in 

 depth) between the iris and the cornea. 



The posterior chamber is the space between the anterior sur- 

 face of the lens and 'the iris. It amounts to a space only at the 

 circumference, the two being in contact at the posterior surface. 



THE CRYSTALLINE LENS is a biconvex, transparent, elastic 

 body, suspended within its capsule, surrounded by the ciliary 

 processes. It is more convex on its posterior surface (anterior 

 surface has an average radius of ten millimetres, the posterior 

 six millimetres), and is received into a hollow depression in the 

 hyaloid membrane. 



It measures about one-third inch in diameter, one-fourth 

 inch in axis, and weighs from four to four and one-half grains. 



Its index of refraction increases from the periphery to the 

 centre, with an average of 1.4371. 



Its composition consists of about 60 per cent, water ; soluble 

 albuminous matter, 35 per cent. ; insoluble albuminous matter, 

 2.5 per cent. ; cholesterine and fat, 2- per cent. 





