'////; NEURONES 



751 



axones, and those whose sheaths are void of myelin, non-medullated axones. 

 However, there is no sharp line of separation between these two varieties, for there 

 may he found axones with sheaths of all decrees of medullation. .Most of the neu- 

 rone.i with non-niedullated and partially mediillated axones belong to the sympa- 

 thetic nervous system. Some sympathetic axones are completely medullated, 

 but their myelin sheaths are always thinner and never so well developed as those 

 of the cerebro-spinal nerves. Certain axones of short course in the central nervous 

 system are non-niedullated. 



In internal structure the cell-body of the neurone consists of a large, spherical, 

 vesicular nucleus with usually one characteristic, centrally placed nucleolus and a 

 cytoplasm continuous into the axone and dendritic outgrowths. The two most 

 interesting structures of the cytoplasm are its granular and iibrillar components. 

 The granules are probably of nutritive significance, and, during the death changes 

 in the cell, show a tendency to collect into clumps of characteristic shape and 

 arrangement which are called tigroid masses or Nissl bodies. These masses are 

 distributed throughout the cytoplasm with the interesting exception that they are 

 not found in the axone or in the immediate vicinity of its hillock of origin. The 

 protoplasmic fibrillae, or spongioplasmic reticulum of the original cell, increase in 



FIQ. 557. SCHEMES SHOWING Two FORMS OF TERMINATION OP AXONES UPON CELL-BODIES OF 



OTHER NEURONES. 

 A. In ventral horn of spinal cord. B. In spinal ganglia. 



thickness during the development of the neurone, and in the sending out of the 

 processes the meshes of the net become so drawn out as to give the appearance of a 

 more or less parallel arrangement of the threads. This appearance sometimes be- 

 comes so manifest in the axone and portions of the cell-body that it has been 

 interpreted as a series of individual and independent threads or neuro-fibrillae. 



Nerve impulses are transmitted from one element to another by means of con- 

 tact rather than by direct anatomical continuity of the two. The axone be'aring 

 the impulse on approaching its termination loses its sheath and breaks up into 

 its numerous terminal twigs, the final of which are called telodendria. Their 

 terminal arrangement assumes forms varying from more compact 'pericellular 

 baskets' to more open arborisations composed of fewer twigs. The twigs of the 

 arborisation or end-brush are either clasped upon the dendrites of the neurone or 

 upon its cell-body direct, as the case may l>e. The general manner of the contact 

 is illustrated in fig 557. It should be mentioned that, contrary to the current belief 

 that impulses are transmitted by -simple contact of neurones.it has been advanced 

 that the terminal twigs frequently penetrate the substance of the cell-body, and it 

 has been held (more recently by Joris) that neuro-fibrillae sometimes pass from 

 one neurone into another. 



The forms of nerve termination in the other tissues of the body are many and 



