76 ANATOMY FOE NUKSES. [CnAr. VII. 



Nerves. Nerves, or as they are sometimes called, nerve- 

 trunks, are whitish cords which arise from the cerebro-spinal 

 axis, and, branching as they go, are distributed to all parts of 

 the body. Every organ and tissue has thus its supply of nerves 

 connecting it with the brain or spinal cord (Fig. 64). 



If we examine a nerve 

 under the microscope, we 

 find that it is composed 

 of nerve-fibres, each fibre 

 being composed of an 

 axone enclosed in a sheath. 

 These fibres are of two 

 kinds, the medullated and 

 the non-medullated. The 

 former consists of a central 

 core, the axone, sur- 

 rounded by a thick sheath 

 of white fatty substance 

 forming what is known 

 as the medullary sheath. 

 Surrounding this is a 

 second sheath, the neuri- 

 lemma, which is very deli- 

 cate and has numerous 

 nuclei situated along its 

 inner surf ace. The second 

 variety of nerve-fibres 

 (the non-medullated) 

 have a similar structure 

 except that in them the 

 medullary sheath is ab- 

 sent. 1 



FIG. 64. DIAGRAM ILLUSTRATING THE GEN- Between the nerve- 

 ERAL ARRANGEMENT OF THE CEREBRO-SPINAL ni 

 SYSTEM. fibres is a small amount 



of connective tissue which 



serves not only to bind the fibres together into bundles, or 

 fumculi, but also to carry to or from the fibres the blood- 

 vessels and the lymphatics necessary for their nutrition. 



1 In the white matter of the brain and spinal cord the fibres are without a 

 neurilemma, and in the gray matter the medullary sheath is also lacking. 



