766 



THE NERVOUS SYSTEM 



dendrite, are given off from opposite poles, the more reticular arrangement about the nucleus is 

 often practically obliterated by the opposing growth stress. 



So manifest does the parallel appearance of the neuro-fibrillae in the processes often become 

 that it has been interpreted as a series of individual and independent fibrils. In the application 

 of gold chloride and similar methods to the neurones of lower forms, the reduced reagent is often 

 precipitated upon the fibrils in parallel, seemingly independent lines. And, assuming the ex- 

 istence of independent fibrils, it has beeiji contended that the neurone is not the functional unit 

 of the nervous system but is itself composed of numerous functional units, individual fibrils, 

 each for the conduction of nerve impulses. More recent and trustworthy methods, however, 

 show that the neuro-fibrillaj retain their original reticular form, the threads anastomosing in 

 all planes, and that the meshes of the net may, in the processes, be so drawn in one direction 

 that a parallel appearance predominates. Further, it is now held that the neuroplasm, or the 

 more fluid substance in which the fibrils lie throughout, is capable, and probably fully as cap- 

 able, of conducting impulses as the fibrils. 



Of the granules in the cytoplasm, the most interesting are those first described in detail by 

 Nissl. These are the most abundant of those in the cell-body and are known as tigroid masses 

 or Nissl bodies. They consist of numerous basophilic granules collected into clumps or masses of 

 varying size. They are known to disappear during fatigue of the nervous system and they are 

 more abundant in animals after a period of rest. They are distributed throughout the cyto- 

 plasm of the cell-body with the interesting exception that they are not found in the axone nor in 

 the immediate vicinity of its place of origin from the cytoplasm, leaving a free region known as 

 the axone hillock. As accumulated masses, they show characteristic shapes and arrangement 



Fig. 609. — Showing Pieces of Axones. 



A. From a cranio-spinal nerve. B. From the spinal cord. C. From the sympathetic, a, 

 axones; m, medullary sheath; n, node of Ranvier; s, neurilemma or sheath of Schwann with 

 occasional sheath-nuclei. 



) 



which are interpreted as signifying the shapes and arrangement of the spaces or meshes they 

 occupy in the reticulum of the neuro-fibrilla>. In cell-bodies of the varieties found in the ventral 

 horns of the spinal cord or in the (;erebral and cerebellar cortex, for example, the masses situated 

 immediately about the iui(;leus are smaller, more numerous and of irregular shape. Nearer and 

 in the beginnings of the dendrites, they are larger and mostly of fusiform or diamond shape. 

 Farther out in the dendrites, they become more and more thin and attenuated; and in the dis- 

 tant readies of th(; dendrites they are invisible or absent. In the cell-body of the spinal ganglion 

 they are of irregular shape, smaller and more numerous throughout the cytoplasm, being slightly 

 smaller and more fliickly placed in the immediate vicinity of the nucleus. In all neurones 

 several hours post-mortein, they appear in fewer and larger masses and it was in this condition 

 that Nissl originally described them in man. Clo.sely examined, the masses of all sizes are found 

 to be accumulations of finer gramiles. Functionally they are supjrosed to be of nutritive signi- 

 ficance, substances in unstable chemical e(iuihlil)rium, energy stcjred in the cytoplasm, capable 

 at need of being split into simpler forms usable in the activities of the neurone. The fact that 

 tigroid masses are ab.sent from the axone hillock, the axone, and the distant reaches of the den- 

 drites inay signify that the substance is (-liicfly present here only in the split and usable form. 

 Also, in th(! axoiif; especially, the neurofibriihe are so (dosely arranged that the meshes of their 

 net here are t(jo small to contain masses of ajjpreciahle size, ("lose examination of the axone 

 hillock and longitudinal .sections of the axone in deeply stained preparations usually show a few 

 very minute l)asopliilic granules. 



Sheaths of the axone. — The great majority of axones acquire sheaths about them which 

 isolate and protect them in their course through other tissues or in company with other axones. 

 A nerve fibre is an axone together with its sheath. In transverse sectioas, the axone comprises 



