162 THE BRAIN OF THE TIGER SALAMANDER 



the presence of a primordium of this structure ('396, p. 604). Some 

 of the smaller tegmental neurons in the vicinity of the VIII roots 

 send axons laterally into tract b, where they divide with ascending 

 and descending branches. Similar fibers are seen to enter this tract at 

 all levels between the V and X roots. Some of these come from the 

 opposite side. The ascending branches of these fibers pass under the 

 auricular gray and recurve dorsalward to arborize within the gray 

 of the rostral face of the auricle. This may represent a vestigial 

 remnant of the large olive of some fishes, here reduced to insig- 

 nificant proportions because of the small size of the cerebellum 

 (Ariens Kappers, Huber, Crosby, '36, pp. 668-89). 



THE LEMNISCUS SYSTEMS 



The less myelinated field of alba of the medulla oblongata between 

 the sensory and motor zones, which I term the "reticular formation," 

 contains the ascending fibers of the lemniscus systems and a neuropil, 

 which receives dendrites of neurons of the underlying gray and also 

 of many neurons of the adjoining sensory and motor zones. This 

 seems to be the field in which patterns of local bulbar reflexes are 

 organized, but its texture and connections have not been satisfac- 

 torily analyzed. From the reticular formation and the overlying sen- 

 sory gray, thick axons, some of which are myelinated, descend as 

 internal and external arcuate fibers to the ventral funiculus. Many of 

 these pass to the motor zone as part of the neuromotor apparatus of 

 bulbar and spinal reflexes. These enter a mixed spino-bulbar and 

 bulbo-spinal tract associated with the spinal lemniscus, as shown in 

 figures 38 and 39. Others, usually thinner fibers, immediately decus- 

 sate in the ventral commissure and then ascend in the lemniscus 

 tracts to higher levels of the sensory zone. 



In the lemnisci of mammals, fibers of the various sensory systems 

 are segregated in separate tracts. In Ambly stoma the arrangement is 

 radically different. Segregation of functional systems is incipient but 

 so little advanced that mammalian names are inapplicable. Under 

 this heading there are included here all ascending fibers from the 

 spinal cord and medulla oblongata, which terminate in the sensory 

 and intermediate zones at higher levels. These are arranged in four 

 groups which take separate courses: (1) the spinal lemniscus complex 

 (Im.sp.), (2) the general bulbar lemniscus (hi.), (3) tr. bulbo-tectalis 

 lateralis (tr.t.b.L), and (4) the ascending secondary visceral-gustatory 

 tract {tr.v.a.). The spino-cerebellar tract is closely associated with the 



