THE EVIDEXCE OF THE RESPIRATORY APPARATUS 1 57 



These observations of van Wijhe lead directly to the following 

 conclusion. In the cranial region there is evidence of a double set 

 of segments, which mav be called somatic and splanchnic. The 

 somatic segments, consisting of the outer skin and the body muscu- 

 lature, are doubly innervated as are those of the spinal cord by a 

 series of ventral motor roots, the oculomotor or lllrd nerve, the 

 trochlear or IVth nerve, the abducens or YIth nerve, and the hypo- 

 glossal or Xllth nerve, and by a series of dorsal sensory roots, the 

 sensory part of the trigeminal or Yth nerve. But the splanchnic 

 segments are innervated by single roots, the vagus or Xth nerve, 

 glossopharyngeal or IXth nerve, facial or Vllth nerve, and trigeminal 

 or Vth nerve, which are mixed, containing both sensory and motor 

 fibres, thus differing markedly from the arrangement of the spinal 

 nerves. 



From this sketch it follows that the arrangement seen in the 

 spinal cord, would result from the cranial arrangement if this third 

 system of lateral roots were left out. Further, since the cranial 

 system is the oldest, we must search in the invertebrate ancestor for 

 a tripartite rather than a dual system of nerve-roots for each segment ; 

 a system composed of a dorsal root supplying only the sensory nerves 

 of the skin-surfaces, a lateral mixed root supplying the system con- 

 nected with respiration with both sensory and motor fibres, and a 

 ventral root supplying the motor nerves to the body-musculature. 



COMPARISON OF THE APPENDAGE NERVES OF LlMULUS AND BrANCHI- 



pus to the Lateral Eoot System of the Vertebrate. 



If the argument used so far is correct, and this tripartite system 

 of nerve-roots, as seen in the cranial nerves of the vertebrate, really 

 represents the original scheme of innervation in the paheostracan 

 ancestor, then it follows that each segment of Limulus ought to be 

 supplied by three nerves— (1), a sensory nerve supplying its own 

 portion of the skin-surface of the prosomatic and mesosumatic 

 carapaces; (2), a lateral mixed nerve supplying exclusively the 

 appendage of the segment, for the appendages carry the respiratory 

 organs ; and (3), a motor nerve supplying the body -muscles of the 

 segment. 



It is a striking fact that Milne-Edwards describes the nerve-roots 

 in exactly this manner. The great characteristic v£ the nerve-roots 



