MEDULLA OBLONGATA OF AMBLYSTOMA 349 



larly establish a very precise regional functional localization in the 

 remaining parts of the white substance. From this it follows that, 

 although in the oblongata as in the cord the dendrites of many 

 neurones of the gray substance spread very widely throughout the 

 white layer, it is possible in all such cases to determine with a 

 high degree of precision the functional character of the various 

 fiber systems with which these dendrites effect synaptic relations. 

 In the medulla oblongata, therefore, we have an exceedingly primi- 

 tive generalized type of histological structure and at the same time 

 a definite pattern of functional localization. The functional factors 

 which determine this localization are represented structurally by 

 an arrangement of peripheral nerve components and central 

 fiber tracts which can readily be correlated with those of the mam- 

 malian nervous system. The functional composition of the 

 cranial nerve roots is known to agree with that of other Ichthyop- 

 sida, and save for the lateral line roots with that of the Amniota 

 as well. Though the central courses of these root components 

 and of the associated correlation tracts are arranged according 

 to a much simpler pattern than in any other vertebrate hitherto, 

 described, nevertheless the homologies of these tracts in higher- 

 brains can in most cases readily be recognized. 



In the embryonic development of the neural tube of vertebrates 

 the sulcus limitans marks the primary boundary between the 

 dorsal afferent and the ventral efferent systems; and in general 

 this boundary is more or less evident throughout life in the amphib- 

 ian oblongata. In fishes and higher vertebrates the efferent 

 centers are further divisible into a ventro-medial somatic column 

 and a ventro-lateral visceral column, the latter including the pre- 

 ganglionic sympathetic neurones for smooth muscles and glands 

 (general visceral efferent system) and the motor nuclei from which 

 are innervated the branchial and masticatory muscles (special 

 visceral efferent system for the striated visceral muscles of the 

 head). In these groups of animals the afferent centers are like- 

 wise divisible into a ventral visceral sensory column (the fasciculus 

 solitarius and associated gray matter) and a dorsal somatic 

 sensory column, the latter receiving the roots of the general cuta- 

 neous, VIII and lateral line nerves. 



