REGIONAL ANALYSIS 43 



U'lie other afferent fibers of the V to X cranial nerves, upon entrance 

 into the brain, are fascicuhitetl according to the functional systems 

 represented, as outlined in the next chapter and shown in figures 7, 9, 

 88, 89, 90. The general cutaneous, lateral-line, and vestibular fibers 

 are arranged in a series of fascicles bordering the external surface; 

 the visceral sensory and gustatory fibers are assembled in a single 

 deeper bundle, the fasciculus solitarius. The marginal fascicles of 

 root fibers are arranged from ventral to dorsal in the following order : 

 general somatic sensory (chiefly cutaneous), vestibular, and, dorsally 

 of these, five or six fascicles of fibers of the lateral-line roots of the 

 VII and X nerves. The details of this arrangement are variable with- 

 in the species and from species to species of urodeles. The fascicles of 

 vestibular and lateral-line roots, together with the underlying gray 

 and the intervening neuropil, comprise the area acusticolateralis. 

 The dorsal part of this, which receives only roots of the lateral-line 

 nerves, is lobulated on the ventricular side. 



Most of these root fibers divide into ascending and descending 

 branches, and each fascicle spans the entire length of the medulla 

 oblongata. Some of the general cutaneous and vestibular fibers ex- 

 tend far down into the spinal cord and upward into the cerebellum 

 (figs. 3, 7). Terminals and collaterals of all these fibers end in a com- 

 mon pool of neuropil, from which secondary fibers go out to effect 

 local connections in the medulla oblongata, to enter the cerebellum, 

 to descend to the spinal cord, and to ascend in the lemniscus systems 

 to the tectum and thalamus. The physiological specificity of the root 

 fibers i.s largely, though not entirely, obliterated at the first synapse 

 in the neuropil of the sensory field, in sharp contrast with the mam- 

 malian arrangement (chap. xi). 



Prom this arrangement of the sensory systems of fibers and their 

 central secondary connections it is clear that the bulbar structure is 

 so organized as to facilitate mass movements of total-pattern type, 

 which may be activated by any one of the exteroceptive or proprio- 

 ceptive systems or by any conibination of these. There is some provi- 

 sion here for local reflexes of the muscles of the head, but the struc- 

 ture indicates that most of these are patterned from higher centers. 

 The central receptive field of the visceral-gustatory system is well 

 segregated from that of the general cutaneous and acousticolateral 

 systems; and this is the structural expression of the fundamental dis- 

 tinction between the visceral and the somatic sensory physiological 

 systems, to which further reference is made on pages G7 and 83. 



