METAMERISM. 253 



nerves, depending- upon the arrangement of these 

 organs. 



Hence, the arrangement of the components in its main 

 outlines, including the terminal centres of the special 

 sensory systems, is a palingenetic character of great per- 

 manence throughout the vertebrata, which is not due to 

 the direct influence of the now present environment. On 

 the other hand, the peripheral courses of these nerves, the 

 number of rami, their individual composition, anastomoses 

 and fusions are to some extent cenogenetic characters to 

 be explained by the pressure of the environment, me- 

 chanics of growth, etc. 



If, now, we frame a conception of the typical spinal 

 nerve of the existing Ichthyopsida, as illustrated by the 

 bony fishes, we find that it receives all of the four primary 

 components and that each of its rami also receives some 

 fibres of each of these four categories. Thus the ventral 

 ramus of the spinal nen^e contains considerable numbers 

 of somatic motor, somatic sensory, viscero-motor and 

 viscero-sensory fibres. The two last components reach 

 their peripheral distribution partially and perhaps wholly 

 through the mediation of the sympathetic nervous system. 

 The medial and dorsal rami contain the two somatic com- 

 ponents in large numbers and the two visceral components 

 in very much smaller numbers, chiefly in the form, no 

 doubt, of vaso- motor and excito-glandular fibres with the 

 corresponding return reflex paths. 



The central connections of the visceral fibres, and especiallj^ of the 

 viscero-sensory fibres in the spinal cord are very obscure. Johnston 

 ('98, p. 597) denies the presence of any viscero-sensory fibres in 

 the spinal nerves. "No sensory fibres of the spinal nerves supply 

 visceral structures. We know of no sensory fibres entering the spinal 

 cord from the sympathetic system." Kolliker ('96, p. 860) makes a 

 similar statement, — "The sensory fibres of the sympathicus all arise 

 from cerebro-spinal fibres and end, like cerebro-spinal sensory 

 elements, in the periphery. The sympathicus possesses no sensory 



