158 THE BRAIN OF THE TIGER SALAMANDER 



large part of the motor field of the medulla oblongata of both sides. 

 How far forward the ascending branches may extend has not been 

 determined; some of them certainly pass beyond the isthmus. Some 

 illustrations of these fibers have been drawn ('396, figs. 42, 46, 51, 57, 

 58, 61, 66, 68). In the medulla oblongata they are mingled with 

 similar fibers of the thalamo-bulbar, pedunculo-bulbar, and teg- 

 mento-bulbar systems ('396, fig. 23). Relatively few of them enter the 

 f. longitudinalis medialis. The extensive and diffuse spread of the 

 terminals of these fibers makes it evident that, if they are employed 

 in any definitely local reactions, the localization is effected by some 

 device other than the arrangement in space of their terminals. 



In mammalian neurology the transverse segment of the brain stem, 

 which lies under the cerebellum, is named the "pons" after its most 

 conspicuous external feature. This name is obviously inappropriate 

 here, where there is no pons or any recognizable primordium of it. 



FIBER TRACTS OF THE MEDULLA OBLONGATA 



The white substance of the medulla oblongata contains many com- 

 pactly arranged, well-myelinated fibers. Some are very coarse and 

 some thinner. Mingled with these, especially in the sensory and inter- 

 mediate zones, there are many unmyelinated fibers. In most prepara- 

 tions the several systems are not clearly fasciculated, so that analysis 

 is difficult in the adult. In larvae elective Golgi preparations have 

 revealed many details, though much remains obscure. Most of the 

 ventromedial fibers are descending. Laterally within and surrounding 

 the reticular formation are fibers of local correlation and the ascend- 

 ing lemniscus systems; and dorsolaterally there are the fascicles of 

 sensory root fibers, together with some correlating systems. Most of 

 the latter appear as arcuate fibers, which are very numerous at the 

 outer border of the gray, with some at intermediate and superficial 

 depths of the alba (figs. 87-90). 



Sensory fibers of the second order are of five sorts: (1) reflex con- 

 nections by arcuate fibers with the bulbar motor zone of the same and 

 the opposite side, either directly or with synapse in the intermediate 

 zone; (2) bulbo-spinal connections, crossed and uncrossed, descend- 

 ing in the ventral and ventrolateral funiculi; (3) bulbo-cerebellar con- 

 nections (see chap, xii); (4) correlation tracts a and h (of Kingsbury), 

 intrinsic to the sensory zone; (5) the lemniscus systems, passing be- 

 tween the sensory zone and higher levels of the same zone, most of 



