THE NERVOUS TISSUE. 53 



Within the gray substance of the nervous centres, the white nerve- 

 fibres either commence from the processes of certain of the nerve- 

 cells, or appear as loops running between those cells. Even in the 

 latter case they may have their origin in cells not immediately under 

 observation. Indeed, the view has lately been advanced, that all 

 nerve-fibres originate in nerve-cells ; and it has further been argued 

 that, most probably, each cell is the centre of one or more complete 

 circuits, a fibre or fibres passing from and returning to it again 

 (Beale). As they pass out from the base of the brain, or from the 

 sides of the spinal cord, the nerve-fibres form bundles of little cords, 

 named funiculi, Fig. 29, a, which are soon gathered into a cluster or 

 nerve-root. In the spinal nerves there are two roots, one posterior, 

 the fibres of which go through a knot or ganglion of gray substance, 

 a, b, and one anterior, the fibres of which go past the ganglion ; both 

 sets join beyond the ganglion to form a single nerve-trunk. In the 

 nerves, and their branches, Fig. 28, a, the tender funiculi are sup- 

 ported in bundles by the neurilemma, a soft sheath, having partitions 

 in it, composed of a form of connective tissue, and continuous with 

 the membranes of the brain or spinal cord. The bundles and even 

 the funiculi often split and interlace, to form nervous plexuses, but the 

 ultimate nerve-fibres, it is believed, do not subdivide, at least in their 

 course, and remain of uniform thickness. 



The nerves appear sometimes to end in loops, sometimes in meshes, 

 but more frequently by free extremities with or without previous sub- 

 division, in the various tissues to which they go. In the muscles, as 



Tig. 30. 



Fig. 30. (Burdach.) Plan of a thin portion of muscle, showing its parallel fibres slightly waved or zig- 

 zag : and a small nerve, n, composed of a bundle of white tubular nerve-fibres, which pass over and 

 amongst the muscular fibres and form returning loops. Magnified 30 diameters. 



shown in Fig. 30, white tubular motor nerve-fibres, once considered 

 terminal, form rather wide loops, which cross amongst the muscular 

 fibres ; but, according to recent observations on the muscles of the 

 lower animals, these loops are by no means the real terminations of 

 the nerves. All agree that the dark-bordered or medullated fibres of 

 the motor nerves give off very fine non-medullated branches, which 

 either end in fine points, or else form a delicate network upon the 

 muscular fibres ; and these fibres are marked by numerous attached 

 nuclei ; and most authorities agree, that they do not penetrate within 

 the sarcolemma. Other appearances have also been described, such 

 as terminal nerve-buds, or blunt knob-like endings of the dark-bor- 

 dered fibres, and more recently the so-called terminal plates of those 



