326 CONSTRUCTION OF THE NERVES. 



be readily seen with the naked eye, and easily dissected out from each other. In a 

 nerve so dissected into its component funiculi, it is seen that these do not run along 

 the nerve as parallel insulated cords, but join together obliquely at short distances 

 us they proceed in their course, the cords resulting from such union dividing in 

 their further progress to form junctions again with collateral cords ; so that in fact 

 the funiculi composing a single nervous trunk have an arrangement with respect 

 to each other similar to that which is found to hold in a plexus formed by the 

 branches of different nerves. It must be distinctly understood, however, that in 

 these communications the medullated nerve-fibres do not join together or coalesce. 

 They pass off from one nervous cord to enter another, with whose fibres they become 

 intermixed, and part of them thus intermixed may again pass off to a third funiculus, 

 or go through a series of f uniculi and undergo still further intermixture ; but 

 throughout all these successive associations the fibres remain individually distinct, 

 like the threads in a rope. 



The nerve-fibres are separated from one another, and supported within the 

 funiculus by delicate connective tissue, the fibrils of which run for the most part 

 longitudinally, appearing in section as fine points (fig. 380). This tissue has been 

 distinguished as the endoneurium by Key and Retzius. It is continuous with the 

 septa which pass in as above mentioned from the innermost lamella of the 

 perineurium, and it serves to support also the capillary blood-vessels which are 

 distributed to the nerve. 



Lying alongside each other, the fibres of a funiculus form a little skein or 

 bundle, which runs in a waving or serpentine manner within its sheath ; and the 

 alternate lights and shadows caused by the successive bendings being seen through 

 the sheath, give rise to the appearance of alternate light and dark cross stripes on. 

 the funiculi, or even on larger cords consisting of several funiculi. On stretching 

 the nerve, the fibres are straightened and the striped appearance is lost. Both the 

 perineurium and endoneurium accompany the nerves in all their divisions, in some 

 cases as far as their peripheral terminations. In the finest branches the perineurium 

 generally becomes reduced to a single connective tissue lamella, covered on both 

 surfaces by endothelial cells. In this condition it is known as the sheath of Henle. 



Both the cerebro-spinal and the sympathetic nerve-trunks are constructed in the 

 manner above described, but the fibres of the cerebro-spinal nerves are chiefly of 

 the white or medullated kind, and contain for the most part fibres of large size, while 

 in nerves belonging to the sympathetic system non-medullated fibres or medullated 

 fibres of very small size greatly preponderate. But very few nerves are composed 

 exclusively of one or the other kind of fibre. 



Vessels and Lymphatics. The blood-vessels of a nerve after dividing into 

 small branches in the epineurium and giving offsets to the groups of fat-vesicles 

 which are there met with, pierce the layers of the perineurium obliquely, being 

 supported by the connective tissue bundles which unite the lamellae, and conducted 

 into the interior of the funiculus along the septa before mentioned. Here they 

 break up into fine capillaries which for the most part run parallel with the fibres, 

 but are connected at intervals by short transverse branches, thus forming a network 

 with long narrow meshes. Some of the capillaries may be observed to form loops. 

 Lymphatic vessels are found in the epineurium, but within the funiculi there are no 

 distinct vessels for the conveyance of lymph. It is found, however, that coloured 

 fluid which is injected by means of a fine cannula into the interior of a funiculus 

 finds its way into the lymphatics of the sheath after passing through the clefts 

 between the lamellae of the perineurium, so that undoubtedly a connection exists- 

 between these perineural clefts and the lymphatic system. 



Course of the nerve-fibres in the nerve-trunks. Neither in their course 

 along the nervous cords, nor in the white part of the nerve-centres, do the 



