CHAPTER VII. 

 THE GENERAL PHYSIOLOGY OF NERVE-TISSUE. 



The nerve- tissue, which unites and coordinates the various 

 organs and tissues of the body and brings the individual into relation- 

 ship with the external world, is arranged in two systems, termed 

 the encephalo spinal or cerebrospinal and the sympathetic. 



The encephalospinal system consists of: 



1. The brain and spinal cord, contained within the cavities of the 



cranium and the spinal column respectively, and 



2. The cranial and spinal nerves. 



The sympathetic system consists of: 



1 . A double chain of ganglia situated on each side of the spinal column 



and extending from the base of the skull to the tip of the coccyx. 



2. Various collections of ganglia situated in the head, face, thorax, 



abdomen, and pelvis. All these ganglia are united by an elab- 

 orate system of intercommunicating nerves, many of which are 

 connected with the cerebrospinal system. 



HISTOLOGY OF NERVE-TISSUE. 



The Neuron. The nerve-tissue has been resolved by the in- 

 vestigations of modern histologists into a single morphologic unit, to 

 which the term neuron has been applied. The entire nervous system 

 has been showri^oTTe but an aggregate of an infinite number of 

 neurons, each of which is histologically distinct and independent. 

 Though having a common origin, as shown by embryologic investi- 

 gations, they have acquired a variety of forms in different parts of the 

 nervous system in the course of development<^-The old conception 

 that the nervous system consisted of two distinct histologic eleme 

 nerve-cells and nerve-fibers, which differed not only in their mode of 

 origin, but also in their properties, their relation to each other, and 

 their functions, has been entirely disproved^ 



The neuron, or neurologic unit, is histologically a nerve-cell, the 

 surface of which presents a greater or less number of processes in 

 varying degrees of differentiation. As represented in figure 43, A, the 

 neuron may be said to consist of: (i) The nerve-cell, neurocyte, or 

 corpus; (2) the axon, or nerve process; (3) the end-tufts, or terminal 

 branches. Though these three main histologic features are every- 

 where recognizable, they exhibit a variety of secondary features in 



103 



