226 



THE PINEAL ORGAN 



the parietal sense organ and the brain in lampreys and reptiles, and 

 the homologous nerve connecting the vestigial sense organ with the 

 roof of the brain in the common frog do afford indirect evidence which 

 justifies the assumption that these fibres are primarily afferent in nature, 

 although their original function of transmitting impulses from the terminal 

 sense organ to the brain has, like the capacity of the sense organ itself to 

 receive these sensations, ceased long ago. 



1. Urodela Caudata 



The pineal organ of Urodeles appears in all examples which have 

 hitherto been examined to show signs of degeneration. Thus in Triton, 

 de Graaf found indications of " fatty " degeneration (probably vacuolation 

 of the cells) ; moreover, the cavity of the vesicle was irregularly sub- 



Ch.-S 



Mo- ' H YP 



Fig. 160. 



A median sagittal section of the head of an embryo of Amblystoma punctatum 

 (Axolotl), showing the relations of the paraphysis, par. ; velum, vel. ; and 

 epiphysis, Ep.; Ch. : notochord; c.p. : commissura posterior; hyp. : hypo- 

 physis ; mo. : mouth. The dorsal sac will be formed between the velum and 

 epiphysis, and the anterior or habenular commissure in front of the epiphysis. 

 (After J. B. Johnstone.) 



divided by partitions into loculi, the septa being produced by folding of 

 its walls. The vesicle is often dorsoventrally compressed, being flattened 

 out between the cranial wall and roof of the brain, and the lumen is 

 thus reduced to a mere slit. In Desmognathus, according to Fish (1895), 

 no trace of a lumen is to be found in the pineal organ of the adult animal. 

 In Typhlotriton and Proteus anguineus, both of which are cave-dwellers 

 and blind, the lateral eyes are also degenerate and buried beneath the 

 skin of the head. In Proteus (Fig. 159) Galeotti found that the epiphysis 

 was a small, pear-shaped structure showing no evidence of secretory 

 function, but he observed that in certain cells near the nucleus pigment 

 granules were present — this being the only place in which pigment was 

 found to be present in the brain of Proteus. This snake-like amphibian, 



