510 NERVOUS SYSTEM. 



characteristic which has given them the name of nucleated nerve-fibres. 

 The diameter of the fibres is about Tr ^ ulr of an inch (3 /*). The nuclei have 

 nearly the same diameter as the fibres and are about -^^ of an inch (20 /A) 

 in length. They are finely granular and present no nucleoli. The fibres are 

 rendered pale by the action of acetic acid, but they are slightly swollen only, 

 and present, in this regard, a marked contrast with the elements of connect- 

 ive tissue. They are found chiefly in the sympathetic system and in that 

 particular portion of this system connected with involuntary movements. 

 They are not usually found in the white filaments of the sympathetic. 



Accessory Anatomical Elements of tlie Nerves. The nerves present, in 

 addition to the different varieties of true nerve-fibres just described, certain 

 accessory anatomical elements common to nearly all of the tissues of the 

 organism, such as connective tissue, blood-vessels and lymphatics. 



Like the muscular tissue, the nerves are made up of their true anatomical 

 elements the nerve-fibres held together into primitive, secondary and terti- 

 ary bundles, and so on, in proportion to the size of the nerve. The primitive 

 fasciculi are surrounded with a delicate membrane, described by Robin, under 

 the name of perinevre, but which had been already noted by other anatomists, 

 under different names, and is now frequently called the sheath of Henle. 

 This membrane is homogeneous or very finely granular, sometimes marked 

 with longitudinal striae, and possessing elongated, granular nuclei. Accord- 

 ing to Ranvier, there are three kinds of nuclei either attached to or situated 

 near the sheath. These are (1) nuclei attached to the inner surface of the 

 sheath ; (2) nuclei belonging to the nerve-fibres within the sheath ; and (3) 

 nuclei of connective-tissue elements near the sheath. Treated with silver 

 nitrate, the sheath presents the borders of a lining endothelium. The sheath 

 of Henle begins 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 mem- 

 brane generally envelops a primitive fasciculus of fibres, branching as the bun- 

 dles divide and pass from one trunk to another, and is sometimes found 

 surrounding single fibres. It usually is not penetrated by blood-vessels, the 

 smallest capillaries of the nerves ramifying in its substance but seldom pass- 

 ing through to the individual nerve-fibres. Within the sheath of Henle are 

 sometimes found elements of connective tissue, with very rarely a few capil- 

 lary blood-vessels in the largest fasciculi. 



The quantity of fibrous tissue in the different nerves- is very variable and 

 depends upon the 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, there is a tolerably strong 

 investing membrane or sheath surrounding the whole nerve and sending into 

 its interior processes which envelop smaller bundles of fibres. This sheath 

 is formed of ordinary fibrous tissue, with small elastic fibres and nucleated 

 connective-tissue cells. These latter may be distinguished from the gelati- 

 nous nerve-fibres by the action of acetic acid, which swells and finally dissolves 

 them, while the nerve-fibres are but slightly affected. 



