138 THE HUMAN BODY 



zations. These are in contact with the branching dendrites of some 

 other neuron. The surfaces of contact between the end arboriza- 

 tion of one neuron and the dendrites of another constitute what is 

 called a synapse. In order for a nerve impulse to pass from one 

 neuron to another it must cross this synapse. 



The Myelin Sheath. All true nerve tissue has a characteristic 

 gray color. This statement applies equally to cell-bodies, den- 

 drites, and axons. Most, but not all, of the long axons of the 

 body are inclosed within sheaths composed chiefly of a substance, 

 myelin, which has a characteristic glossy white color. The myelin 

 sheath where present does not inclose the axon throughout its 

 entire length; near the cell-body and again near its termination 

 the axon is not inclosed. Surrounding the myelin sheath, or, 

 where it is absent, the axon itself, is a delicate membrane, the 

 neurilemma. The myelin sheath is made up of short seg- 

 ments which are separated one from another by the nodes of 

 Ranvier. 



The myelin sheath is not composed of living cells and so does 

 not contain nuclei. The neurilemma, however, is a living mem- 

 brane; scattered along it at intervals are nuclei. The function 

 of the myelin sheath is not known. Perhaps the most satisfactory 

 suggestion that has been offered is that it serves as an insulator 

 to keep the nerve impulse within its own axon and prevent its 

 escape to adjacent ones. 



Axons which are inclosed in myelin sheaths are spoken of as 

 medullated or myelinated nerve-fibers. 



It is the presence of myelin sheaths that gives to certain parts 

 of the nervous system their characteristic white appearance. All 

 " white matter " is made up of medullated axons. " Gray matter," 

 on the other hand, is made of cell-bodies and dendrites, together 

 with some non-medullated axons. 



The Central and Peripheral Nervous Systems. In a preceding 

 paragraph was pointed out the analogy between the nervous sys- 

 tem and a telephone system. That part of the nervous system 

 corresponding to the telephone " exchange," to which sensory 

 neurons lead and from which motor neurons spring is called the 

 central nervous system. It consists of the brain and spinal cord. 

 (The analogy between the central nervous system and a telephone 

 exchange should not be pushed too far, for the central nervous 



