68 HUMAN PHYSIOLOGY. 



surface of the heel bone. Its action is to extend the foot and to 

 raise the weight of the body in walking and running. On the front 

 of the leg are numerous muscles e. g., tibialis anticus, peroneus 

 longus, etc., the action of which is to flex the foot and to antagonize 

 the gastrocnemius. 



PHYSIOLOGY OF 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 arranged anatomically into two systems, 

 termed the encephalo or cerebro-spinal and the sympathetic. 



The encephalo or cerebro-spinal 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 cerebro-spinal system. 



HISTOLOGY OF NERVE TISSUE. 



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

 gations of modern histologists into a single morphologic unit, to which 

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

 been shown to be 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 investigations, 

 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 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. 



