GENERAL ANATOMY. 499 



nection in the nerve centres, and in the different organs and tissues they supply ; 

 the former are called their central, the latter their peripheral terminations. 



As to the mode in which the nerve fibres are disposed in the nervous centres, it 

 is probable that many originate from nerve corpuscles, in the manner before men- 

 tioned; others probably form simple loops. As to the more exact mode of 

 connection of the nerve fibres with the nerve corpuscles, it appears that more 

 commonly as the fibre approaches the vesicle, the white substance of Schwann 

 gradually disappears, and the tubular membrane expands, so as to envelope the 

 corpuscle ; the sheath, contracting at the opposite side of the corpuscle, is again 

 continuous with the tubular sheath of the nerve fibre, a prolongation from the 

 granular substance of the corpuscle extending for some distance along each, part 

 of the nerve tube, and taking the place of the usual elements of the nerve fibre. 

 Whether this relation of nerve fibres to ganglion-corpuscles is common to all 

 kinds of nerve fibres, has yet to be determined. 



In the peripheral distribution of the nerves, small bundles of nerve fibres com- 

 monly form delicate plexuses ; these, dividing, give oft' the primitive fibres, which 

 are disposed of in various ways in different tissues. 1. Occasionally the elemen- 

 tary fibres are disposed in loops, as has been found in the internal ear, in the 

 papillas of the tongue and of the skin, in the tooth pulp, and in striped muscular 

 tissue ; each fibre, after issuing from a branch in a terminal plexus, runs over or 

 through the substance of the tissue, and, turning back, joins the same or a neigh- 

 boring branch, in which it probably passes back to a nervous centre. 2. Some- 

 times each ultimate nerve fibre divides into several branches, which spread out in 

 the substance of the tissue, as is seen in the retina, in the muscular tissue of the 

 frog and lower vertebrata. 3. Sometimes the ultimate nerve fibres form minute 

 pkxuses, as in certain serous membranes, viz., the peritoneum, and in the pia mater 

 of the brain and cord. 4. Not uncommonly the nerve fibres terminate by free 

 ends, as is seen in the Pacinian corpuscles, and in some of the papilla3 of the skin. 

 5. Occasionally, the nerve fibres are brought into direct connection with nerve- 

 corpuscks, as in the retina and in the lamina spiralis of the internal ear. 



Some nerve fibres have no peripheral termination. Gerber has shown, that 

 nerve fibres occasionally form loops, by their junction with a neighboring fibre in 

 the same fasciculus, and return to the cerebro-spinal centre without having any 

 peripheral termination. These he considers to be sentient nerves, appropriated 

 exclusively to the nerve itself, the nervi nervorum, upon which the sensibility of 

 the nerve depends, and quite exclusive of the sensation produced by an impression 

 made at the peripheral end of the nerve. These fibres bear some analogy to those 

 met with in the back part of the optic commissure, where a set of fibres passes 

 from one optic tract across the commissure to the opposite tract, having no com- 

 munication with the optic nerve ; also in the communications formed between the 

 cervical nerves and spinal accessory and descendens noni, the nerve fibres forming 

 an arch connected by each extremity with the cerebro-spinal centre, and having 

 no peripheral termination. 



Again, some nerve fibres would appear to have no central connection with the 

 cerebro-spinal centre, as those forming the most anterior part of the optic con> 

 missure. These inter-retinal fibres, as they are called, commence in the retina, 

 pass along the optic nerve, and across the commissure to the optic nerve and 

 retina of the opposite side. 



The point of connection of a nerve with the brain or spinal cord is called, for 

 convenience of description, its origin or root. If the fasciculi of which the nerve 

 is composed should all arise at or near one point, or along one tract, the root is 

 called single. If, on the contrary, the fasciculi divide into two separate bundles, 

 which are connected at two different points with any part of the cerebro-spinal 

 centre, such nerve is said to have a double origin, or to arise by two roots, each of 

 which may have a separate function, as in the spinal nerves. The point where 

 the separate fasciculi of a nerve are connected to the surface of the cerebro-spinal 



