CHAP. XI.] PERIPHERAL NERVOUS END-ORGANS. 459 



developed, whilst the other nervous layers are absent. At the so- 

 called macula lutea, and especially at its central depression, tho 

 fovea centralis^ cones are found to the exclusion of rods ; at the 

 periphery of the macula lutea cones are found, each surrounded by a 

 circle of rods, whilst over the rest of the retina the cones are found 

 sparsely distributed amongst the rods. 



Variation ^ e ^acillary l a j er f the retina does not always 



in the distr- possess both rods and cones ; in some animals we find 

 button of rods r ods and in others cones, or where both are present their 

 and cones in relative number varies. Thus the following animals 

 the retinae of nave no cones : the ray, the shark, the sturgeon, the 

 cifsseTof kat, ^ e nec % en g> tne mole. The following animals 



animals. nave no rods: lizards, serpents, tortoises, and perhaps 



all reptiles. All mammals have more rods than cones; 

 nearly all birds have more cones than rods, though in the owl, which 

 is a nocturnal bird, the cones are very few. 



Chemical composition of the Retina as a whole. 



In consequence of the scanty material at the chemist's disposal 

 little is known as to the general composition of the retina. The 

 reaction of the retina is said to be acid. According to C. Schmidt 1 

 the retina, besides containing albumin, yields, on boiling, gelatin and 

 mucin. Its alcoholic extract yields a body which gives a crystal- 

 lizable compound with platinum chloride and which smells of tri- 

 inethylamine, doubtless due to the decomposition of neurine. As 

 Klihne remarks, we may on general grounds surmise that the retina 

 contains the same bodies as the central nervous systems. 



Whilst the living retina is perfectly transparent, at death it 

 becomes opaque, doubtless in consequence of the coagulation of some 

 proteid constituent. 



General Chemical facts relating to Rods and Cones. 

 Chemical -^ e inner segments of both rods and cones are corn- 



structure of posed of protoplasm which during life is possessed of 

 the inner marvellous transparency ; after death this becomes 



limbs of the opaque and presents granular deposits, nuclei, and in 

 some cases spherical, lenticular, or paraboloid, highly 

 refractile bodies. 

 rho^i^i The outer limbs of the rods are composed of an 



dnemicai . . . - 1 . 



structure of external envelope, which agrees, m physical characters 

 the outer and in its power of resisting various agents, with neuro- 



limbs of the keratin. This external envelope encloses contents which 

 morphologically appear as little disks which are sepa- 

 rated by an intermediate substance ; it is impossible to distinguish 

 between the chemical characters of these two kinds of substances. 

 Ktilme has pointed out that the contents of the envelopes consist of a 

 mixture of proteid bodies and of substances soluble in alcohol and 



1 See Kiihne, Chemie der Netzhaut. " Hermann's Handbuch, Vol. in. Part 1, p. 239. 



