142 Journal of Comparative Neurology. 



not only the mouth but also the other parts of the face and the 

 mucous membranes of the head. Just this afferent nerve of 

 the face teaches us a good lesson for the general arrangement 

 of the sensory-motor mechanisms. We find that it spreads 

 over the neurones for the movement of the jaws, the movement 

 of the facial muscles, the tongue and even into the segments of 

 the neck (by the 'ascending' root). Thus a prick of the cheek 

 can be responded to directly by a movement of the jaw, of the 

 muscles of the face, and of the entire head by the muscles of 

 the neck. 



These segments constitute the hind-brain, an important ac- 

 cessory organ of which we shall recognize presently in the cer- 

 ebellum. 



4. The mid-brain enlargement contains as a segmental 

 mechanism the optic nerve and the nerves for the remaining 

 muscles of the eyeball : the optic segment. 



5. The forebrain contains only an afferent nerve-apparatus, 

 the olfactory which is only in indirect communication with the 

 various motor neurones. 



This is a summary of the neuro-muscular segments which 

 constitute the human body. How are they connected ? 



A study of the lowest vertebrate, the Amphioxus, may 

 show how the various segments of the body can be very simply 

 united for conjoint action. There are of course first the ground 

 bundle-elements or intersegmental neurones. In the head end 

 of the neural tube there are a few very large cells sending big 

 fibers towards the caudal segments ; and in the caudal end there 

 are also a few large cells making connection with the segments 

 of the head end. The cells are sufficient, together with the 

 intermediate cells between the neighboring segments, to repre- 

 sent the coordination of movement of the Amphioxus. Such 

 a simple mechanism would not suffice for the higher vertebrates. 

 The number of such long connecting cells would be immense, 

 considering the variety of complex movements of which we are 

 capable. On the plan of the Amphioxus our nervous system 

 would consist of the anterior horn cells and their fibers to the 

 muscles, of the afferent ganglion cells and their fibers into the 



