THE EVIDENCE OF THE RESPIRATORY APPARATUS 153 
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- 
rent 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 vertebree 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 
