MEDULLA OBLONGATA OF AMBLYSTOMA 369 



contiguous to these roots appears to possess a certain loosely 

 organized physiological unity, for the neurones of this gray sub- 

 stance tend to spread out within the stratum album of this region 

 only and very often to reach practically all parts of it. This 

 portion of the oblongata (including both the white and the gray 

 layers) may, therefore, properly be called the area acustico- 

 lateralis. Nevertheless individual neurones are sometimes seen 

 to send dendrites into definite restricted parts only of this area 

 and other dendrites into the spinal V root or even into the under- 

 lying tegmentum (figs. 27, 28, 29, 35). 



The dendrites of the neurones of the area acustico-lateralis 

 which are related to the roots of the lateral line nerves may pass 

 directly outward into the white layer, but more commonly they 

 run for considerable distances in the marginal zone of the gray 

 substance and give thick contorted branches into the white layer, 

 where they end in dense tufts among special tracts, each neurone 

 thus usually effecting synaptic relations with one or more roots of 

 both the lateralis VII and the lateralis X and perhaps also with 

 the VIII roots (figs. 34, 36) and with correlation tract a or h. 

 Other forms of these endings are seen in figures 27, 32, 35. The 

 most important synapses are probably effected in the marginal 

 zone of the white substance related to the respective fiber tracts 

 of the white substance adjacent, for here are found the densest 

 tufts of dendritic arborizations. 



The axones of these sensory neurones of the second order are 

 directed downward and inward in the marginal zone of the gray 

 substance. As arcuate fibers, after decussation in the ventral 

 commissure, some effect various forms of reflex connection within 

 the oblongata and a larger number enter the bulbar lemniscus, or 

 tractus octavo-tectalis et thalamicus {Im. of the figures). This 

 tract is comparable with the fasciculus lateralis of the oblongata of 

 fishes and in part with the lateral lemniscus of mammals. It is 

 a compact bundle of fibers of medium size, heavily myelinated in 

 the adult, which ascends to the midbrain and largely terminates 

 by free arborizations in the stratum album of the tectum mesen- 

 cephali. A smaller number of its fibers, however, continue 

 forward to terminate in the thalamus. 



