968 NUCLEI OF CRANIAL NERVES. [BOOK m. 



been accompanied by a considerable dislocation of parts. We 

 must accordingly expect to find the indications of a segmental 

 arrangement greatly obscured on the one hand by transposition, 

 and on the other by fusion. 



The twelfth or hypoglossal nerve is one whose nature seems 

 fairly simple. It is in function exclusively an efferent nerve. The 

 large cells, with conspicuous axis-cylinder processes, which charac- 

 terize its nucleus, are exactly like those of the anterior horn of 

 the spinal cord which give origin to the fibres of an anterior root. 

 The nucleus moreover in its position corresponds to part of the 

 anterior horn of the spinal cord, if we take into account the 

 shifting involved in the decussation of the pyramids, and in the 

 new developments of the bulb. If we compare Fig. 109 with any 

 section of the cord, we see that the hypoglossal nerve corresponds 

 to an anterior root of the spinal cord, but that the fibres, after, 

 leaving the cells from which they take origin, traverse in the 

 former a large tract, and in the latter case a small tract of tissue. 

 Whether the whole nerve corresponds to the fibres of several 

 segments fused together, or to those of one segment spread out 

 longitudinally, is for our present purposes of secondary importance. 



Recognizing the hypoglossal nerve as the homologue of a 

 spinal anterior root, we may go on to claim the nuclei of the third 

 and fourth nerves as similar groups of cells of the anterior horn, 

 giving rise to anterior roots. The position of the nuclei, the 

 character of the cells, the function of the fibres, all support this 

 view. The case is perhaps not so clear as that of the hypoglossal 

 nerve, since there are reasons for thinking that these nerves have 

 undergone in the course of evolution greater changes than has the 

 hypoglossal nerve ; still these reasons do not oppose the above 

 conclusion. 



The nucleus of the exclusively motor sixth nerve does not 

 exactly correspond to those of the third and fourth in position; 

 but we may probably place it in the same series with them. 

 Thus we have in succession, the third, fourth, sixth, and twelfth 

 nerves, with their respective nuclei, as the anterior roots of nerves 

 of their several segments. 



In the fifth nerve, the dislocation and fusion spoken of above 

 has introduced difficulties. The motor nucleus, with the fibres of 

 the motor root to which it gives origin, has by some been con- 

 sidered 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 



