44 THE BRAIN OF THE TIGER SALAMANDER 



Otherwise, there is httle histological evidence of precise localization 

 of function in the medulla oblongata. The visceral and somatic sen- 

 sory fields are cross-connected within the sensory zone, and they 

 converge into a common sensory field at the bulbo-spinal junction. 

 Proprioceptive adjustments are made throughout the spinal cord, 

 medulla oblongata, cerebellum, and tectum; and each of these re- 

 gions evidently plays a different role in the adjustments. Arcuate 

 fibers connect all parts of the sensory zone with the motor zone of the 

 same and the opposite side, and many of these divide into long- 

 descending and ascending branches, thus activating extensive areas 

 of the intermediate and motor zones. 



The motor field of the medulla oblongata and the intimately re- 

 lated reticular formation contain the complicated apparatus by 

 which the nuclei of the motor nerves are so interconnected as to act 

 in groups, each of which may execute a series of co-ordinated actions 

 in patterns determined by these connections. The tissues of the motor 

 tegmentum, which effect this analysis of motor performance, are so 

 intricately interwoven that it has not been possible to recognize the 

 components of the several functional systems, and further analysis 

 of this field is desirable. 



4. CEREBELLUM 



The cerebellum is small and very simply organized, but the chief 

 structural features of the mammalian cerebellum are present except 

 the pontile system, which is totally lacking. The urodele cerebellum 

 consists of three major parts: (1) the median body, activated from 

 the spinal cord, trigeminal nerve, and tectum (figs. 1, 3, 10); (2) the 

 lateral auricles, which are enlargements of the anterior ends of the 

 sensory zones of the medulla oblongata (figs. 7, 91); and (3), ven- 

 trally of the body of the cerebellum, a nucleus cerebelli, which is the 

 primordium of the deep cerebellar nuclei of mammals (figs. 10, 32, 

 91). 



This analysis of cerebellar structure is based on the comprehensive 

 studies of Larsell ('20-'47) and Dow ('42), whose descriptions of 

 Amblystoma, published in 1920 and 1932, I have confirmed in all 

 respects. It should be noted that my definition of the amphibian 

 auricle includes more than Larsell's, for he includes in this structure 

 only the vestibular and lateral-line components. I find that these 

 terminals and the related field of neuropil are so intimately related 

 with the terminals of the trigeminus, the visceral-gustatory system, 

 and lemniscus fibers that their segregation is not practical anatomi- 



