THE NERVOUS TISSUE. 1119 



fasciculi, which are bound together in a common membranous investment, and 

 constitute the nerve. 



In structure the common membranous investment, or sheath of the whole nerve, 

 which is called the epineurium, as well as the septa given off from it, and which 

 separate the fasciculi, consists of connective tissue, composed of white and yellow 

 elastic fibres, the latter existing in great abundance. The tubular sheath of the 

 funiculi, called the perineurium, consists of a fine, smooth, transparent membrane, 

 which may be easily separated, in the form of a tube, from the fibres it encloses ; 

 in structure it consists of connective tissue, which has a distinctly lamellar arrange- 

 ment, consisting of several lamellae, separated from each other by spaces containing 

 lymph. The nerve-fibres are held together and supported within the funiculus by 

 delicate connective tissue called the endon curium. It is continuous with septa 

 which pass inward from the innermost layer of the perineurium, and consists of a 

 ground-substance in which are embedded fine bundles of fibrous connective tissue 

 which run for the most part longitudinally. It serves to support the capillary 

 vessels, which are arranged so as to form a network with elongated meshes. The 

 cerebro-spinal nerves consist almost exclusively of the medullated nerve-fibres, 

 the non-medullated existing in very small proportions. 



The blood-vessels supplying a nerve terminate in a minute capillary plexus, 

 the vessels composing which pierce the perineurium and run, for the most part, 

 parallel with the fibres ; they are connected together by short, transverse vessels, 

 forming narrow, oblong meshes, similar to the capillary system of muscle. Fine 

 non-medullated nerve-fibres accompany these capillary vessels, vaso-motor fibres, 

 and break up into elementary fibrils, which form a network around the vessel. 

 Horsley has also demonstrated certain medullated fibres as running in the epineu- 

 rium and terminating in small spheroidal tactile corpuscles or end-bulbs of Krause. 

 These nerve-fibres, which Marshall believes to be sensory, and which he has termed 

 nervi nervorum, are considered by him to have an important bearing upon certain 

 neuralgic pains. 



The nerve-fibres, as far as is at present known, do not coalesce, but pursue an 

 uninterrupted course from the centre to the periphery. In separating a nerve, 

 however, into its component funiculi, it may be seen that they do not pursue a 

 perfectly insulated course, but occasionally join at a very acute angle with other 

 funiculi proceeding in the same direction ; from this, branches are given off, to 

 join again in like manner with other funiculi. It must be distinctly understood, 

 however, that in these communications the nerve-fibres do not coalesce, but merely 

 pass into the sheath of the adjacent nerve, become intermixed with its nerve-fibres, 

 and again pass on, to become blended with the nerve-fibres in some adjoining 

 funiculus. 



Nerves, in their course, subdivide into branches, and these frequently commu- 

 nicate with branches of a neighboring nerve. 



The communications which take place between two or more nerves form what 

 is called a plexus. Sometimes a plexus is formed by .the primary branches of the 

 trunks of the nerves as the cervical, brachial, lumbar, and sacral plexuses and 

 occasionally by the terminal funiculi, as in the plexuses formed at the periphery 

 of the body. In the formation of a plexus the component nerves divide, then 

 join, and again subdivide in such a complex manner that the individual funiculi 

 become interlaced most intricately ; so that each branch leaving a plexus may 

 contain filaments from each of the primary nervous trunks which form it. In the 

 formation also of smaller plexuses at the periphery of the body there is a free 

 interchange of the funiculi and primitive fibres. In each case, however, the 

 individual filaments remain separate and distinct, and do not inosculate with one 

 another. 



It is probable that through this interchange of fibres the different branches 

 passing off from a plexus have a more extensive connection with the spinal cord 

 than if they each had proceeded to be distributed without such connection wit. 

 other nerves. Consequently the parts supplied by these nerves have more extended 



