90 TEXT-BOOK OF PHYSIOLOGY. 



their discoverer, occur at regular intervals along the course of the nerve, 

 separating it into a series of segments. The portion between the nodes is 

 termed the internodal segment. It has been suggested that in consequence 

 of the absence of the myelin at these nodes, a free exchange of nutritive 

 material and decomposition products can take place between the axis- 

 cylinder and the surrounding plasma. Beneath the neurilemma in each 

 internodal segment there is a large nucleus surrounded by a small amount of 

 granular protoplasm. 



The End Tufts. The end-tufts or terminal organs are formed by the 

 splitting of the axon into a number of filaments, which remain independent 

 of one another and are free from the myelin investment. The histologic 

 peculiarities of the terminal organs vary in different situations, and in many 

 instances are quite complex and characteristic. In peripheral organs, as 

 muscles, glands, blood-vessels, skin, mucous membrane, the tufts are in 

 direct histologic and physiologic connection with their cellular elements. In 

 the brain and spinal cord the tufts are in more or less intimate relation 

 with the dendrites of adjacent neurons. 



The neurons in their totality constitute the neuron or nerve tissue. 

 From the fact that they are arranged both serially and collaterally into a 

 regular and connected whole, they collectively constitute a system known as 

 the neuron or nerve system. 



The neurons composing the spinal and cranial nerves are represented in 

 Fig. 44, which are connected peripherally by their terminal branches with 

 muscles on the one hand and with epithelium of skin, mucous membrane, etc., 

 on the other hand. In the spinal cord the terminal branches of the afferent 

 neuron come into histologic and physiologic relation with the dendrites of a 

 second neuron, the axonic process of which in many instances ascends the 

 cord to different levels or even as far as the brain, where its terminal branches 

 come into relation with the dendrites of still another neuron, the axonic 

 process of which is in turn connected with neurons in the cortex of either 

 the cerebrum or cerebellum. The surfaces of the body are thus brought 

 into relation with the cerebral and cerebellar neurons. The neurons 

 arranged in this serial manner constitute the afferent side of the nerve 

 system. 



In a similar way the efferent neurons of the spinal and cranial nerves are 

 brought into relation with the cortex of the cerebrum. Large pyramidal- 

 shaped neurocytes situated in specialized regions of the cortex of the cere- 

 brum send their axonic processes down through the brain and cord. As they 

 approach their destination the terminal branches become related histo- 

 logically and physiologically with the dendrites of the neurons composing 

 the cranial and spinal nerves. The cortex of the cerebrum is thus brought 

 into relation with the general musculature of the body. The neurons 

 arranged in this serial manner constitute the efferent side of the nerve system. 



Neurons, moreover, are grouped into more or less complexly organized 

 masses, termed organs, which in accordance with their locations may be 

 divided for convenience into central and peripheral organs. 



The Central Organs of the Nerve System. The central organs con- 

 sist of the encephalon and spinal cord, contained within the cavities of the 

 head and spinal column respectively. They consist of neurons arranged 



