PHYSIOLOGICAL ANATOMY OF THE NERVOUS TISSUE. 



569 



on, in proportion to the size of the nerve. The primitive fasciculi are surrounded by a 

 delicate membrane, described by Robin under the name ofperinevre, but which had been 

 already noted by other anatomists under different names. This membrane is homogeneous 

 or very finely granular, sometimes marked with longitudinal striae, and possessing elon- 

 gated nuclei, finely granular, from ^^ to -^^ of an inch in 

 length by from -g-^7 to 3~oVo f an mcn wide. The thickness of 

 the membrane is from 1 2 1 6 to 8 ^ of an inch. It commences at 

 the point where the nerve-fibres emerge from the white portion of 

 the nervous centres, and it extends to their terminal extremities, 

 being interrupted by the ganglia in the course of the nerves. This 

 membrane generally envelops a primitive fasciculus of fibres, branch- 

 ing as the bundles divide and pass from one trunk to another ; but 

 it is sometimes found surrounding single fibres. It is not usually 

 penetrated by blood-vessels, the smallest capillaries of the nerves 

 ramifying in its substance but seldom passing through to the indi- 

 vidual nerve-fibres. Within the perinerve, are sometimes found 

 elements of connective tissue, with very rarely a few capillary 

 blood-vessels in the largest fasciculi. 



The amount of fibrous tissue in the different nerves is very 

 variable and depends upon the conditions to which they are sub- 

 jected. In the nerves within the bony cavities, where they are 

 entirely protected, the fibrous tissue is very scanty ; but, in the 

 nerves between muscles, we find a tolerably strong investing mem- 

 brane or sheath surrounding the whole nerve and sending pro- 

 cesses into its interior, which envelop smaller bundles of fibres. 

 This sheath is formed of inelastic fibres, with small elastic fibres 

 and nucleated connective-tissue fibres. These latter may be distin- 

 guished from the gelatinous nerve-fibres by the action of acetic 

 acid, which swells and finally dissolves them, while the nerve- 

 fibres are but slightly affected. 



The late researches of Sappey have shown that the structure 

 of the fibrous sheath of the nerves possesses certain important 

 anatomical peculiarities. The greatest part of this membrane is composed of bundles 

 of white inelastic tissue, interlacing in every direction ; but it contains also numerous 

 elastic fibres, adipose tissue, a net-work of arteries and veins, and "nervi-nervorum," 

 which are to these structures what the vasa-vasorum are to the blood-vessels. The 

 adipose tissue is constant, being found even in extremely emaciated persons. 



The vascular supply to most of the nerves is rather scanty. The arteries break up into 

 a plexus of very fine capillaries, arranged in oblong, longitudinal meshes surrounding the 

 fasciculi of fibres ; but they rarely penetrate the perinerve, and they do not usually come in 

 contact with the ultimate nervous elements. The veins are rather more voluminous and 

 follow the arrangement of the arteries. It is not certain that the nerves in their course 

 contain lymphatics ; at least these vessels have never been demonstrated in their substance. 



Branching and Course of the Nerves. The ultimate nerve-fibres in the course of the 

 nerves have no connection with each other by branching or inosculation. A bundle of 

 fibres frequently sends branches to other nerves and receives branches in the same way ; 

 but this is simply tbe passage of fibres from one sheath to another, the ultimate fibres them- 

 selves maintaining throughout their course their integrity and their individual physiologi- 

 cal properties. This view with regard to the course of the fibres in the nerves is held by 

 nearly all anatomists. The nerve-fibres do not branch or inosculate except at the point 

 where they change their character just before their termination. The branching and inos- 

 culation of the ultimate nerve-fibres will be considered in connection with the very inter- 

 esting and important question of their ultimate distribution to muscles and sensitive parts. 



Fm. 175. Fibres of Re- 

 mak; magnified 800 

 diameters. (Robin.) 



With the gelatinous fibres, 

 are seen two of the or- 

 dinary, dark-bordered 

 nerve-fibres. 



