THE NERVOUS SYSTEM 



343 



form gray matter; meduUated fibers appear white. That the medullary 

 sheath serves at least for insulation is indicated by the fact that nervous 

 impulses are conveyed more rapidly in meduUated than in non-medullated 

 nerves. 



Within the central nervous system of vertebrates, neurites lack the 

 neurilemma sheath, but are usually meduUated. The presence of a 

 neurilemma is, therefore, not essential to the secretion of this fatty myelin 

 sheath. Most peripheral nerves are meduUated, but the medullary sheath 

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

 of Amphioxus and of cyclostomes are not meduUated. This primitive 



MOTOR 

 GANGUON CELL 



A. DIAGRAM OF A REFLEX ARC 



Fig. 305. — Diagram of a nervous arc. In ,4 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. 



condition is retained by the sympathetic nerves and plexuses of higher 

 forms. Primitive ganglion ceUs 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 multipUcation in the number of possible func- 

 tional relations. In ontogenesis, as in phylogenesis, aU 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 gangUonic masses. In this 

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



