472 



COMPARATWE ANATOMY 



does not appear until after the neurolemma is differentiated. The nerves 

 of amphioxus and of cyclostomes are not medullated. This primitive 

 condition is retained by the sympathetic nerves and plexuses of higher 

 forms. Primitive ganglion cells like those of coelenterates occur in 

 vertebrates only in the parasympathetic plexuses associated with the 

 alimentary canal. 



The forms assumed by neurons in vertebrates are varied; and in 

 general, the more complex the animal the more complex its neurons. 

 Complication in form usually involves an increase in the number of 

 dendrites, and denotes a multiplication in the number of possible func- 



A. DIAGRAM OF A REFLEX ARC 



Fig. 390. — Diagram of a nervous arc. In A three neurons — afferent, intercalary, 

 and efferent — are shown in their relations to one another and to the skin and muscle. 

 The intercalary neuron is located in the gray matter of the spinal cord. B is an enlarged 

 section of a nerve fiber. 



tional relations. In ontogenesis, as in phylogenesis, all the processes of 

 neurons are formed as processes of primarily simple neuroblasts. 



Nerve cells manifest a tendency not only to spin out elongated proto- 

 plasmic processes so as to connect with various parts of the body and with 

 one another, but also to form- plexuses and ganglionic masses. In this 

 way, the complex nervous systems of higher animals have been built up. 

 The first nervous connexions appear to have been between skin and 

 muscle, by means of neurosensory cells, so that a motor response to 

 external stimulus is made possible. So simple an arrangement as this is, 

 however, rarely found, even in worms and molluscs. Usually at least 

 two nerve cells are involved in a reflex action, a neurosensory receptor and 

 a motor ganglion cell which connects with muscle or gland. Even more 

 frequently, a third or association cell is interpolated between the receptor 

 and the motor cells. These association cells may multiply to form a chain 

 of neurons within the central nervous system. Such complications were 



