46 INTRODUCTION TO NEUROLOGY 



the cytoplasm of the cell and extending out into the larger den- 

 drites (see Figs. 6, 7). These masses were first carefully investi- 

 gated by Nissl, who devised a special staining method for that 

 purpose; they are, accordingly, often called the Nissl bodies, and 

 sometimes tigroid bodies. They never occur in the axon nor 

 in a special conical protuberance of the cell body (the axon 

 hillock) from which the axon arises (see Fig. 5, ah, and Fig. 6, c). 



The neurofibrils are very delicate strands of denser protoplasm 

 found in all parts of the neuron except the nucleus. They are by 

 many regarded as the specific conducting elements of the neuron, 

 though the evidence for this is not conclusive. They ramify 

 throughout the cytoplasm (Fig. 12), passing through the cell 

 body from one process to another. 



The longer nerve-fibers are usually enveloped by a thick white 

 glistening sheath of myelin, a fat-like substance secreted by the 

 nerve-fibers themselves. This myelin sheath, or medullary 

 sheath, is a part of the neuron with which it is related and the 

 fibers which possess it are called myelinated or medullated fibers; 

 these fibers compose the white matter of the brain and a large 

 part of the peripheral nerves (see Fig. 5). There may be, in ad- 

 dition, in the case of the peripheral nerves an outer sheath, the 

 neurilemma (primitive sheath or sheath of Schwann). This is a 

 thinner nucleated membrane, not a part of the neuron to which 

 it is attached, but formed from surrounding cells. 



The function of the myelin sheath has often been regarded as 

 simply that of an insulating substance to prevent the overflow 

 and loss of the nervous impulse conducted by the axon, but 

 there is some evidence that this sheath plays an important part 

 in the chemical processes involved in the act of nervous conduc- 

 tion. The neurilemma is likewise often spoken of as a protecting 

 membrane. Whether it has any other function in the normal 

 life of the nerve-fiber is unknown ; but if a nerve-fiber is by acci- 

 dent severed from its cell body, it is known that the nuclei of 

 the neurilemma play a very important part in the degeneration 

 and regeneration of the severed fiber and the restoration of its 

 normal function. 



As has been suggested, nerve-fibers cut off from their cell 

 bodies immediately die and degenerate. But in the case of 

 peripheral nerves the neurilemma nuclei do not die; and, appa- 



