156 



THE HUMAN BODY. 



external one is vascular, and in contact with the choroid; the 

 internal one, very important in a physiological point of view, 

 is the membrane of Jacob. It is composed of cylinders, or 

 rods, joined together like the stakes of a palisade, perpendi- 

 cular to the plane of the membrane, and forming by their 

 free extremities a mosaic, each microscopic division of which 



Fig. 35- Rods of Jacob under the microscope. 



A. Rods of Jacob. D. Points of the retinal mosaic receiv- 



B. Their extremities forming surface of ing different luminous rays. 



retina. E. Points receiving each two different 



C. Retinal mosaic formed by the rods. rays. 



is about o'ooi of a line in diameter according to Robin, 

 and 0*0008 of a line according to Helmholtz; and repre- 

 sents a section of a rod. We shall see what part these ter- 

 minal points play in vision. 



Vitreous body. The cavity of the globe of the eye in its 

 three posterior quarters is occupied by a substance completely 

 translucent, the vitreous humor. According to most anatom- 

 ists, it is contained in an envelope called the hyaloid mem- 

 brane. The vitreous humor and the hyaloid together consti- 

 tute what is called the vitreous body, which is perfectly adapted 

 to the retina throughout its whole extent, and in front takes 

 the form of the posterior surface of the crystalline. According 

 to those anatomists who admit the existence of the hyaloid, 



