106 



NORMAL HISTOLOGY, 



Fig. 92. 



the axis-cylinder, and may, therefore, be regarded as a product of 

 that greatly extended arm of the cytoplasm of the nerve-cell. 



The amount of medullary substance present in different nerves 

 varies greatly. Sometimes it is so slight as to be hardly distin- 

 guishable. In other cases its thickness considerably exceeds the 

 diameter of the axis-cylinder. It is present within the spinal cord 

 and brain, although not enclosed in neurilemma in those situations. 



At the peripheral ends of the nerves, on 

 the contrary, it usually disappears before 

 the neurilemma. 



The individual nerve-fibres are isolated 

 only at their extremities. Throughout 

 most of their course they are collected 

 into bundles, forming the "nerves" of 

 the body. Within these bundles the nerve- 

 fibres are held together by fibrous tissue 

 in the following manner : a delicate areolar 

 tissue containing their vascular supply lies 

 between the individual fibres. This fibrous 

 tissue is called the " endoneurium." The 

 nerve-fibres, thus held together, are aggre- 

 gated into bundles, called "funiculi," which 

 are surrounded by sheaths of still denser 

 fibrous tissue, rich in lymphatic spaces, 

 which are called the " perineurium." This 

 perineurium on its inner surface becomes 

 continuous with the endoneurium just de- 

 scribed. The funiculi, enclosed by their 

 perineurium, are, in turn, held together 

 by an areolar sheath, which has received 

 the name, "epineurium," and forms the 

 outer covering of the nerve. 



The funiculi do not run a distinct course 

 throughout the length of the nerve, but 

 give off nerve-bundles, enclosed in peri- 

 neurium, which join other funiculi ; the 

 nerve-fibres themselves do not, however, 

 anastomose with each other. 

 (h) The gray, or non-medullated, nerve-fibres are, as their name 

 implies, destitute of medullary substance. They consist of an axis- 



NVrve-fibres from the sympa- 

 thetic system. (Key and Ret- 

 zius.) All the fibres except 

 that marked m are non-med- 

 ullated. The fibre m has an 

 incomplete medullary sheath, 

 n, n, nuclei of the neurilemma. 

 These are surrounded by a 

 small amount of cytoplasm, 

 which is not clearly repre- 

 sented in the figure. 



