ACCESSORY ANATOMICAL ELEMENTS OF THE NERVES 461 



Accessory Anatomical Elements of the Nerves. The nerves present, 

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

 certain accessory anatomical elements common to nearly all the tissues 

 of the organism, such as connective tissue, bloodvessels and lymphatics. 



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

 tomical elements the nerve-fibres held together into primitive, sec- 

 ondary and tertiary bundles, and so on, in proportion to the size of the 

 nerve. The primitive fasciculi are surrounded with a delicate membrane 

 called the sheath of Henle. This membrane is homogeneous or very 

 finely granular, sometimes marked with longitudinal striae, and possess- 

 ing elongated granular nuclei. There are three kinds of nuclei either 

 attached to or situated near the sheath. These are (i) 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. Silver nitrate discloses the borders of a lining endo- 

 thelium. 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 membrane usually envelops a primi- 

 tive fasciculus of fibres, branching as the bundles divide and pass from 

 one trunk to another. It is sometimes found surrounding single fibres. 

 It usually is not penetrated by bloodvessels, the smallest capillaries of 

 the nerves ramifying in its substance but seldom passing through to the 

 individual nerve-fibres. Within the sheath of Henle are sometimes 

 found elements of connective tissue, with very rarely a few capillary 

 bloodvessels in the largest fasciculi. 



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

 depends on 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. 



The greatest part of the fibrous sheath of the nerves is composed of 

 bundles of white inelastic tissue, interlacing in every direction ; but it 

 contains also many elastic fibres, adipose tissue, a network of arteries 

 and veins, and "nervi nervorum," which are to these structures what 

 the vasa vasorum are to bloodvessels. 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 fine capillaries, arranged in oblong 



