THE NOTOCHORD AND ALIMENTARY CANAL 447 
this direction; thus the musculature of the oral chamber has been 
derived directly from the musculature of the prosomatic appendages ; 
the muscles which move the eyes from the prosomatic and meso- 
somatic dorso-ventral somatic muscles ; the longitudinal body-muscles 
from the dorsal longitudinal somatic muscles of the arthropod ; the 
muscles of respiration from the dorso-ventral muscles of the meso- 
somatic appendages. 
In all these cases we have been dealing with striated musculature 
and consequently with only the motor nerves of the muscle; but the 
gut posterior to the pharyngeal or respiratory chamber contains 
unstriped instead of striped muscle, and is innervated by two sets of 
nerves, those which cause contraction and are motor, and those which 
cause relaxation and are inhibitory. It is by no means certain that 
these two sets of nerves possess equal value from a morphological 
point of view. The meaning of an inhibitory nerve is at present 
difficult to understand, and in this instance, is rendered still more 
doubtful owing to the presence of Auerbach’s plexus along the whole 
length of the intestine—an elaborate system of nerve-cells and nerve- 
fibres situated between the layers of longitudinal and circular muscles 
surrounding the gut-walls, which has been shown by the recent 
experiments of Magnus, to constitute a special enteric nervous system. 
One of the strangest facts known about the system of inhibitory 
nerves is their marked tendency to leave the central nervous system 
at a different level to the corresponding motor nerves, as is well 
known in the case of the heart, where the inhibitory nerve—the 
vagus—arises from the medulla oblongata, while the motor nerve—the 
augmentor or accelerator—leaves the spinal cord in the upper thoracic 
region. It is very difficult to obtain any idea of the origin of such a 
peculiarity ; I know of only one suggestive fact, which concerns the 
innervation of the muscles which open and close the chela of the 
crayfish, lobster, etc. These muscles are antagonistic to each other, 
and both possess inhibitory as well as motor nerves. The central 
nervous system arrangements are of such a character that the contrac- 
tion of the one muscle is accompanied by the inhibition of its opposer, 
and the nerves which inhibit the contraction of the one, leave the 
central nervous system with the nerves which cause the other to 
‘contract. Thus the inhibitory and motor nerves of either the abduc- 
tor (opener) or adductor (closer) muscles of the crayfish claw do not 
leave the central nervous system together, but in separate nerves. 
