CHAPTER XVII. 



THE NERVOUS SYSTEM. 



I. THE STRUCTURE OF THE NERVOUS ELEMENTS. 



NERVOUS tissue is found under the microscope to consist essentially 

 of two main elements, namely, of nerve fibres and nerve cells. When the 

 nerve fibres are collected together into bundles they form nerve trunks or 

 nerves. When nerve cells are collected together they form nerve ganglia, 

 but in such ganglia nerve-fibres are also invariably found. 



A. Nerve Fibres. 



Varieties. In most nerve-trunks two kinds of fibres are mingled, 

 called (A) medullated or white fibres, and (B) non-medullated or gray 

 fibres. 



(A.) Medullated Fibres. Each medullated nerve-fibre is made up 

 of the following parts: (1.) An external sheath called the primitive 

 nerve sheath, or nucleated sheath of Schwann; (2.) An intermediate or 

 packing substance known as the medullary sheath, or white substance of 

 Schwann; and (3.) internally the axis-cylinder, primitive band, axis 

 band, or axial fibre. 



Although these parts can be made out in nerves examined some time 

 after death, in a recent specimen the contents of the nerve-sheath appear 

 to be homogeneous. But by degrees they undergo changes which show 

 them to be composed of two different materials. The internal or cen- 

 tral part, occupying the axis of the tube (axis-cylinder), becomes gray- 

 ish, while the outer, or cortical portion (white substance of Schwann), 

 becomes opaque and dimly granular or grumous, as if from a kind of 

 coagulation. At the same time, the fine outline of the previously trans- 

 parent cylindrical tube is exchanged for a dark double contour (Fig. 309, 

 B), the outer line being formed by the sheath of the fibre, the inner by 

 the margin of curdled or coagulated medullary substance. The granu- 

 lar material shortly collects into little masses, which distend portions of 

 the tubular membrane; while the intermediate spaces collapse, giving 

 the fibres a varicose, or beaded appearance (Fig. 309, c and D), instead 

 of the previous cylindrical form. The whole contents of the nerve- 



