232 



NERVOUS SYSTEM OF VERTEBRATES. 



such as the frog and crocodile the somatic sensory lateral lobes 

 are greatly increased in size. This increased size is not due chiefly 

 to a greater number of sense organs or to a richer cutaneous 

 innervation, but to an increase in the secondary tracts from the 

 cutaneous, auditory and optic centers to the cerebellum, and to 

 a greater number of Purkinje cells whose fibers go to motor nuclei. 

 In other words, the cerebellum in the more highly organized 

 amphibia and reptiles shows an increase in the correlating mechan- 

 isms in comparison with the primary sensory apparatus. 



The further steps in the development of the correlating mechan- 

 ism in birds and mammals are not well understood for lack of 

 comparative studies. A careful study of the structure and fiber 



Mesenceph. 



Cerebellum 

 Fiss. postn. 



Fiss. prase 



esenceph. 

 Fiss. prima 



Bulb.olf. 

 Fiss.rhin. Para 



Tuberc. acust. 

 Floe Velum 



A Plex. chorioid. 



Fig. 1 1 8. A, the left lateral aspect of the brain of a pouch specimen of Dasy- 

 urus viverrinus; B, median sagittal section of the cerebellum of the same brain. 

 After G. Elliot Smith. Fiss. praec., fissura praeculminis; Fiss. postn , fissura 

 postnodularis; Fiss. sec., fissura secunda; Floe., flocculus; Para floe., paraflocculus. 



connections of the cerebellum in a series of forms including repre- 

 sentatives of monotremes, marsupials, bats, rodents and carnivores 

 is much needed in order to show the course of evolution of the 

 extremely complex cerebellum of man. No connected account 

 of this can be given at present, but it is evident that a larger num- 

 ber of specialized centers come into relation with the cere- 

 bellum in man than in fishes. At the same time the structure 

 of the cerebellum itself has become more complex. This com- 

 plexity of structure and of anatomical connections indicates the 



