THE EVIDENCE OE THE RESPIRATORY APPARATUS 1 53 



Now, the striking fact of the vertebrate segmental nerves consists 

 in this, that, as far as their structure and the tissues which they 

 innervate are concerned, the cranial segmental nerves are built up on 

 the same plan as the spinal ; but as far as concerns their exit from 

 the central nervous system they are markedly different. A large 

 amount of ingenuity, it is true, has been spent in the endeavour to 

 force the cranial nerves into a series of segmental nerves, which 

 arise in the same way as the spinal by two roots, of which the ven- 

 tral series ought to be efferent and the dorsal series afferent, but 

 without success. We must, therefore, consider the arrangement of 

 the cranial segmental nerves by itself, separately from that of the 

 spinal nerves, and the problem of the origin of the vertebrate seg- 

 mental nerves admits of two solutions — either the cranial arrange- 

 ment has arisen from a modification of the spinal, or the spinal from 

 a simplification of the cranial. The first solution implies that the 

 spinal cord arrangement is older than the cranial, the second that 

 the cranial is the oldest. 



In my opinion, the evidence of the greater antiquity of the cranial 

 region is overwhelming. 



The evidence of embryology points directly to the greater phylo- 

 genetic antiquity of the cranial region, for we see how, quite early in 

 the development, the head is folded off, and the organs in that 

 region thereby completed at a time when the spinal region is only at 

 an early stage of development. We see how the first of the trunk 

 somites is formed just posteriorly to the head region, and then more 

 and more somites are formed by the addition of fresh segments poste- 

 riorly to the one first formed. We see how, in Ammoccetes, the first 

 formed parts of the skeleton are the branchial bars and the basi- 

 cranial system, while the rudiments of the vertebra? do not appear 

 until the Petromyzon stage. We see how, with the elongation of the 

 animal by the later addition of more and more spinal segments, 

 organs, such as the heart, which were originally in the head, travel 

 down, and the vagus and lateral-line nerves reach their ultimate 

 destination. Again, we see that, whereas the cranial nerves, viz. the 

 ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and 

 vagus nerves, are wonderfully fixed and constant in all vertebrates, 

 the only shifting being in the spino-occipital region, in fact, at the 

 junction of the cranial and spinal region, the spinal nerves, on the 

 other hand, are not only remarkably variable in number in different 



