THE GRAY MATTER. 613 



which we shall presently speak of as the " red nucleus," but apparently 

 making no connections with these structures, and pierce the median edge of 

 the pes, emerging (Fig. 131, (?.) on the surface to the median side of each 

 crus. As we shall see later on, this nerve is now exclusively efferent, what- 

 ever it may have been in more primitive beings. We shall also see later on 

 that impulses starting from the cerebrum of one side pass to the nerve of the 

 other side, that is to say, decussate ; and this is also the case with the other 

 efferent cranial nerves. The fibres which appear to take origin from the 

 nerve-cells of the nucleus do not cross over after emerging from the nucleus, 

 but keep to the same side ; there is no distant decussation as in the case just 

 noted of the fourth nerve. There are, however, fibres (Fig. 137, III'.) which 

 leaving the nucleus cross the median raphe from one side to the other, and 

 these possibly are the paths for the decussation of the impulses ; but they 

 may be fibres passing from the crus across the raphe to the nucleus. This 

 nerve has special relations with the optic tract, but of these we shall speak 

 when we come to deal with the functions of the nerves. 



537. In attempting to understand the nature and relations of these 

 cranial nerves, it must be borne in mind that, while morphological studies 

 lead us to believe that, as the vertebrate body has been developed out of an 

 invertebrate ancestry, so the brain of the vertebrate has arisen by a series 

 of modifications from the nervous structures placed at the head and around 

 the mouth of an invertebrate, the same studies teach us that such an evo- 

 lution has been accomplished by means of profound changes. We have, for 

 instance, reason to think that the mouth of the vertebrate does not corre- 

 spond to the mouth of the invertebrate, but is a new structure, whose appear- 

 ance has 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 con- 

 spicuous axis-cylinder processes, which characterize 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 develop- 

 ments of the bulb. If we compare Fig. 132 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 their 

 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 corre- 

 spond 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. 



