ACCESSORY ANATOMICAL ELEMENTS. 27 



neons or very finely granular, sometimes marked with longi- 

 tudinal striae, and possessing elongated nuclei, finely granular, 

 from ^Vo- to TOTRT f an inc h in length by from ^Vir to 

 TTOTF f an mc ^ wide. The thickness of the membrane is 

 from I2 ooo to -g^nj-Q of an inch. It commences at the point 

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

 nervous centres, and 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, branching as the bundles divide and pass from one 

 trunk to another; but it is sometimes found surrounding 

 single fibres. An important anatomical fact connected with 

 this membrane is that it is never penetrated by blood-vessels, 

 the smallest capillaries of the nerves ramifying in its sub- 

 stance, but never passing through to the individual nerve- 

 fibres. Within the perinerve, are sometimes found ele- 

 ments of connective tissue, but never any other of the ac- 

 cessory anatomical elements of the nerves. 1 



The amount of fibrous tissue in the different nerves is very- 

 variable and depends upon the external conditions to which 

 they are subjected. 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 toler- 

 ably strong investing membrane, or sheath surrounding the 

 whole nerve and sending processes into its interior, which 

 envelop smaller bundles of fibres. This sheath is formed of 

 inelastic fibres with small elastic fibres and nucleated con- 

 nective-tissue fibres. These latter may be distinguished 

 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 struct- 

 ure 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 



1 LlTTRE ET ROBIX, loc. tit. 



