614 THE BRAIN. 



In the fifth nerve the dislocation and fusion spoken of above has intro- 

 duced difficulties. The motor nucleus, with the fibres of the motor root to 

 which it gives origin, has by some been considered as homologous to the 

 series just described ; but it is at once obvious that we cannot look upon this 

 great fifth nerve as corresponding to one spinal nerve, with its anterior and 

 posterior root, great as the superficial resemblance seems to be. The features 

 of the remarkable ascending root forbid this. The fibres of this root may be 

 traced back, as we have said, to the very beginning of the bulb, and, indeed, 

 into the spinal cord beyond ; as far as can be ascertained, they are not in an 

 obvious and direct manner connected with nerve-cells along their course ; 

 but the bundle of 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 the 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 

 toward the hinder parts), we can hardly suppose that it belongs to a single 

 segment, or is the complement of the motor root alone ; in it, most prob- 

 ably, 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 dis- 

 tinct 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, author- 

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

 said enough to show 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, glosso-pharyngeal, and bulbar accessory, 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 dia- 

 gram (Fig. 138) is shaded accordingly. It may, perhaps, be compared to 

 the sensory nucleus of the fifth. Thus, the ascending root, or fasciculus soli- 

 tarius, 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 glosso-pharyngeal fibres, and 

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



The nucleus ambiguus contains large conspicuous cells and we may prob- 

 ably 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 cate- 

 gory, and further class with them the motor nucleus of the fifth, looking 

 upon all three as so many detached portions of gray matter, corresponding 

 to some part of the anterior horn of the spinal cord. Whether they are 

 exactly homologous to the hypoglossal nucleus, and their fibres to simple 

 anterior roots, is not so clear. 



