CHAPTER VIII. 



THE GENERAL PHYSIOLOGY OF NERVE-TISSUE. 



The Nerve-tissue. The nerve-tissue, which unites and coordinates 

 the various organs and tissues of the body and brings the individual into 

 relationship with the external world, is conventionally 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 chain of ganglia situated on each side of the spinal column and extend- 



ing 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 elaborate 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 investigations 

 of modern histologists into single morphologic units, to which the term 

 neurons has been applied. The entire nerve system has been shown to be 

 but an aggregate of an infinite number of neurons, each of which is histologic- 

 ally distinct and independent. Though having a common origin, as shown 

 by embryologic investigations, they have acquired a variety of forms in 

 different parts of the nerve system in the course of development. The old 

 conception that the nerve system consisted of two distinct histologic elements, 

 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 Fig. 44, 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 everywhere recognizable, they exhibit a variety of 

 secondary features in different situations in accordance with peculiarities of 

 function. 



The Nerve-cell. The nerve-cell, or body of the neuron, presents a 

 variety of shapes and sizes in different portions of the nerve system. 

 Originally ovoid in shape, it has acquired, in course of development, pecu- 

 liarities of form which are described as pyramidal, stellate, pear-shaped, 

 spindle-shaped, etc. The size of the cell varies considerably, the smallest 



