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 rneso- 

 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. 



