GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 347 



FIQ. 86. 



system, where they were first discovered, and where they often consti- 

 tute a majority of all the nerve fibres present. In the trunks and 

 branches of the cerebro-spinal system they are much less numerous, 

 but vary in proportion in different nerves and in different species of 

 animals. In all cases, nerves consisting mainly or exclusively of medul- 

 lated fibres have an opaque, white, glistening aspect, due to their mye- 

 line ; while those containing non-medullated fibres are grayish or semi- 

 transparent, according to the proportion of these fibres in their tissue. 



All the medullated nerve fibres lose their myeline and become non- 

 medullated shortly before their termination in the muscular tissue or 

 the organs of sensibility ; and they are also non-medullated at and near 

 their termination in the gray matter of the brain and spinal cord. In 

 these situations the nerve fibre is reduced to a simple axis cylinder, by 

 which it is connected with the peripheral and central organs of the 

 nervous system. 



Course and Mutual Relation of the Nerve Fibres. In the white 

 substance of the brain and spinal cord the nerve fibres form continuous 

 tracts, lying in close apposition with 

 each other, enveloped only by a deli- 

 cate granular and finely fibrillated 

 intervening material. But on emerg- 

 ing from the bony cavities of the 

 cranium and vertebral canal, they 

 are collected into distinct bundles, 

 each invested by a lamellated sheath 

 of fibrous connective tissue, and en- 

 closed in a larger compound mass by 

 a common fibrous sheath or "neuri- 

 lemma." Such a compound bundle 

 is called a nerve, and the fibres which 

 it contains are distributed, after a 

 longer or shorter transit, usually to 

 associated organs or adjacent regions 

 of the body. 



So far as our observation extends, 

 the individual nerve fibres, as a rule, 

 are continuous and independent, from 

 their origin in the nervous centres to 

 within a short distance of their pe- 

 ripheral termination. When a nerve 

 divides into several branches, or 

 when adjacent nerves communicate 

 by inosculation, as in the cervical, 

 brachial, or lumbar plexuses, it is because certain fibres leave those 

 with which they were associated and pursue a different course. A 

 nerve which originates, for example, from the spinal cord, and passes 

 down the arm to the muscles and integument of the hand, contains at 



DIVISION OF A NERVOUS BRANCH (a), into its 

 ultimate fibres, 6, c, d, e. 



