310 



THE MAINTENANCE OF THE INDIVIDUAL 



SECTION C. 



MECHANISMS OF SENSATION AND 

 CO-ORDINATION 



The Morphological Unit — The Neuron 



In order to get at the secret of control of skeletal, muscular, and 

 nervous systems, it is necessary to examine the various nervous devices 

 foimd throughout the animal kingdom which have been developed as 

 co-ordinators. All animals, except perhaps the protozoa, are built 

 up of a number of essentially similar cell units. The complexity of 

 the adjustment device is directly related to the way these units are 



put together, as well as to the 

 actual number of the units 

 comprising the nervous system. 

 Since the fundamental unit of 

 structure of the nervous system 

 is the nerve cell (neuron), we will 

 do well to examine it further. 

 A typical neuron consists of a 

 cell body and two kinds of out- 



r 



direction 

 oP impulse'. 



5— deridrite^ 



nucleus 



Icellbody 



.■naksd. a^on 

 ;<.jCoUateral 



nucleus of— . 

 yieurilemma 



..medullary 5beatb growths, the many branched 



dendrites which receive impulses. 



indicate^ _ . . 



6reat lendtVi fi^ , ro 



^ ^ Vi..r2odsofl?arJvier 



1 



and the elongated axon, that 

 conducts messages away from 

 the body of the cell, and ter- 

 minates in the end organs. The 

 ''naked" axon is characteristic 

 of the gray matter of the cen- 

 tral nervous system. Around 

 many of the axons is a thin, 

 membranous protective cover- 

 ing, called the neurilemma, or 

 Schwann^s sheath. This is liv- 

 ing tissue as shown by the 

 nuclei scattered through it, and 

 by the fact that it may be 

 regenerated after injury. Neu- 

 rons of this latter type are found in most invertebrate nervous 

 systems, in some of the prochordates, and in some of the peripheral 

 nerves. In parts of the central nervous system of vertebrates the 

 neurilemma is replaced by segments of white fatty substance, called 



^-neurilemma 



\j2rmir2a\ branched 



A typical multipolar nerve cell. 



