256 



ANATOMY OF VERTEBRATES. 



201 



subjoined cut (fig. 201, b) gives Dr. Jacob's illustration of this 

 membrane as partly reflected from the back of the retina. In the 

 centre of the retina and axis of vision is a speck which retains its 

 transparency when the rest of the nervous expansion has become 

 opaque after death ; this speck is margined by a yellowish tint ; 

 and in the dead eye one or more short delicate folds pucker the 

 contiguous retina. It was regarded as a natural perforation by 

 its discoverer, and has been called the ' foramen of Soemmerring : ' 

 it is a modification of the retina. The relative position of the 

 4 macula centralis ' to the termination of the optic nerve, whence 

 the branches of the arteria centralis diverge, is shown in fig. 

 201, a. The retinal neurine terminates at the posterior margin 

 of the ciliary body. The vitreous humour, which mainly main- 

 tains the sphericity of the 

 eye, consists of water, 98 *40 ; 

 chloride of sodium with a lit- 

 tle extractive matter, 1*42 ; 

 albumen, 0*16 ; a substance 

 soluble in water, 0*02. It 

 is lodged in the cells of the 

 hyaloid membrane, receives 

 in an anterior depression 

 the crystalline lens, fig. 202, 

 a, from the circumference 

 of which it is extended to 

 the anterior extremities of the ciliary processes, shows their im- 

 pressions at c, and bounds the posterior chamber of the aqueous 



humour. The cellular structure of 

 the part of the hyaloid at the cir- 

 cumference of the lens when demon- 

 strated by inflation or injection, pro- 

 duces the appearance shown at b, 

 called by its describer Petit, ' canal 

 godronne: ' the folds of the hyaloid in 

 relation to the ciliary processes form 

 the '■ corona ciliaris,' ib. c. In the 



human crystalline lens the anterior 

 is to the posterior convexity as 4 to 

 3 : the transverse diameter is from 4 

 to 4| lines, the thickness or axis 

 is about 2 lines. The degrees of 

 convexity of both surfaces vary at different periods of life. 

 In fio\ 203, A shows the lens of a six-months' foetus, B, of 

 a child of six years, c, of an adult of middle age : after fifty 



a. Back of retina, showing macula centralis and porus 



opticus. 

 B. * Jacob's membrane' reflected from the retina, cv". 



Vitreous humour with hyaloid membrane and 



lens, showing the ' canal of Petit' and 



corona ciliaris; magn. cv". 



