SECTION III 

 THE PHYSIOLOGY OF NERVE 



CHAPTER XI 

 THE NEURON AND ITS CONDUCTING PATHS 



The Neuron. — The entire nervous system is an aggregate of an 

 infinite number of neurons which are held together by a nervous 

 supporting framework or neurogha, but many parts of it also contain 

 cells showing a different histological character. Thus, it is found 

 that the spinal cord and the cerebrum are enveloped by protective 

 membranes which are made up of connective tissue, and contain in 

 addition blood vessels and lymph channels for nutritive purposes. The 

 element which we are chiefly interested in at this time is the neuron 

 or nerve-cell. It consists of a cell-body and its processes, the latter 

 being divided into dendrites and the axon or neurit. 



In spite of the fact that the neuron is developed from a single 

 embryonic unit, known as a neuroblast, the adult cell presents a great 

 variety of forms. It may be pyramidal, oval, round, stellate or 

 spindle-shaped, and its size may vary from 10-1 50/x. The cyto- 

 plasm of each cell embraces a nucleus with its nucleolus, and a proto- 

 plasm which is granular in some places and striated in others. The 

 latter contains numerous rounded bodies which stain deeply with 

 methylene-blue and other dyes. These are the so-called Nissl's 

 granules. Especially at the poles of the cell the cytoplasm is arranged 

 in a distinct fibrillar manner, and is extended outward in the form of 

 long processes, which, as has just been stated, are classified as dendrites 

 and axons. The former divide very frequently and irregularly, and 

 do not pass far away from the cell-body. Theii terminals are generally 

 beset with short stubby processes, known as the lateral buds or gem- 

 mules. They impart a peculiar uneven appearance to these processes. 



Each cell-body usually gives rise to several dendrites but only to 

 one axon. The latter is distinguished from the former by its much 

 greater length, its uniform caliber, its smoothness and the greater di- 

 rectness of its course. It gives off very few branches, which are desig- 

 nated as collaterals, and exhibits a hyaline consistency. The dendrites, 

 on the other hand, are not sharply differentiated from the cell-body 

 unless they are long, when they may also acquire a hyaline appearance. 



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