CHAP, ii.] THE BRAIN. 969 



fibres clings, as we have seen, to the gelatinous substance of the 

 posterior horn of the spinal cord and to the continuation of this 

 along the bulb, and the fibres are lost in this structure. The 

 root, therefore, as we have said, corresponds very closely to part 

 at least of the posterior root of a spinal nerve, and, though the 

 matter has not yet been experimentally proved, we may infer 

 that the trophic centres of these fibres are to be found in the 

 cells of the Gasserian ganglion. 



But if this ascending root be of the nature of a posterior root 

 (and we may incidentally remark that the term ascending has 

 been unhappily chosen, since, if it be an afferent root, the direction 

 of the impulses which it carries will be a descending one, namely 

 from the entrance in the pons towards the hinder parts), we can 

 hardly suppose that it belongs to a single segment, or is the com- 

 plement of the motor root alone ; in it, most probably, the posterior 

 fibres of several segments are blended together. Further, we may 

 perhaps infer that the other fibres of the sensory root which end 

 directly in what we have called the sensory nucleus, are in nature 

 quite distinct from the fibres of the ascending root ; and if so, 

 difficulties arise as to the nature and homologies of the nucleus 

 in question. These, however, we must not discuss here, nor can 

 we enter into the question of the nature of the descending root, 

 concerning the fibres of which, as we have said, authorities differ 

 as to whether they pass into the motor or sensory root. We have 

 said enough to shew that this fifth nerve is extremely complex, 

 and that its apparent conformity to a simple spinal nerve is in 

 reality misleading. 



The fibres of the vagus, glossopharyngeal, and bulbar accessor}', 

 taken together, are partly efferent, partly afferent. The combined 

 nucleus of these three nerves, the cells of which are small and 

 devoid of conspicuous axis-cylinder processes, is usually regarded 

 as a sensory nucleus, and in the diagram, Fig. 115, is shaded 

 accordingly. It may perhaps be compared to the sensory nucleus 

 of the tifth. Thus, the ascending root, or fasciculus solitarius, 

 presents many analogies with the ascending root of the fifth, and 

 we are led to regard this as, like it, a gathering of certain afferent 

 fibres of the posterior roots of several segments ; in its case also 

 the term ascending is misleading. But there are many difficulties 

 in connection with this nucleus, as with the fifth. We must not 

 enter into a detailed discussion concerning them, but may remark 

 that we have here perhaps to deal with complexities due to the 

 fact that certainly many vagus and glossopharyngeal fibres, and 

 probably some of those of the fifth, are splanchnic in function. 



The nucleus ambiguus contains large conspicuous cells and we 

 may probably regard it as a motor nucleus, especially of the vagus 

 fibres. We may also perhaps place it and the nucleus of the 

 seventh nerve in the same category, and further class with them 

 the motor nucleus of the fifth, looking upon all three as sa 



