brain; and they diminish in the latter as they approach the cortical substance. The fibres of the nerves of special sense are smaller than the average, in every part of their course.

375. The gelatinous fibres cannot be shown to consist of the same variety of parts as the preceding; no tubular envelope can be distinguished; and the white substance of Schwann seems wanting. They are flattened, soft and homogeneous in their appearance, bearing a considerable resemblance to the unstriped Muscular fibres; and, like them, they contain numerous shell-nuclei, which are arranged with tolerable regularity. These nuclei are brought into view by acetic acid, which dissolves the rest of the fibre, leaving them unchanged. The gelatinous fibres are usually of smaller size than the tubular, their diameter averaging between the 1-6000th and the 1-4000th of an inch; and they sometimes show a disposition to split into very delicate fibrillæ. Being of a yellowish-gray color, they have been sometimes distinguished as gray fibres. These two classes of fibres have been supposed to be essentially distinct in character and office; the "tubular" having been regarded as ministering to the Animal functions of sensation and motion; and the "gelatinous" as connected with the Organic or nutritive operations. The facts which will be presently stated (§ 388) regarding their origin, however, as well as their joint existence in almost every nerve, are decidedly adverse to this view; and we shall find reason to consider them as differing chiefly in grade of development. In fact it appears that the very same fibre may be "tubular" in one part of its course, and "gelatinous" in another.

the

376. The Nerve-fibres appear to run continuously, from one extremity of a nervous cord to the other, without anything like union or anastomosis; each ultimate fibre probably having its distinct office, which it cannot share with another. The fasciculi, or bundles of fibres, however, occasionally intermix and exchange fibres with each other; and this interchange may take place among either the fasciculi of the same trunk, or among those of different trunks. Its object is evidently to diffuse among the different branches the endowments of a particular set of fibres. Thus we shall hereafter see that, in all the Spinal Nerves of Vertebrata, one set of roots ministers to sensation, and another to motion; the sensory fibres are principally distributed to the skin, and the motor fibres to the muscles; but every branch contains both sensory and motor fibres, which are brought together by the interlacement of those connected with both sets of roots. In the head, we have some nervous trunks which have sensory roots alone; and others which have motor roots only; these in like manner acquire each other's functions in some degree by an interchange of filaments,-the sensory trunk receiving motor fibres, and the motor trunk receiving sensory fibres. An interchange of this kind, upon a very extensive scale, takes place between the Cerebro spinal system, whose ganglionic centres are the brain and spinal cord, and the Sympathetic system, whose centres consist of a number of scattered ganglia. The former sends a large number of fibres into the latter, by the twigs of communication near the origins of the Spinal nerves, as well as by their connecting branches; whilst

the latter sends a smaller number of fibres into the former, these being chiefly of the gelatinous kind.

377. Sometimes we find the fasciculi of several distinct trunks united into an extensive plexus; the sole object of which appears to be, to give a more advantageous distribution to fibres, which all possess corresponding endowments.. Thus the brachial plexus mixes together the fibres arising by five pairs of roots, on either side, from the spinal cord; and sends off five principal trunks to supply the arm. Now, if each of these trunks had arisen by itself, from a distinct segment of the spinal cord, so that the parts on which it is distributed had only a single connexion with the nervous centres, they would have been much more liable to paralysis than they are. By means of the plexus, every part is supplied with fibres arising from each of the five segments of the spinal cord; and the functions of the whole must, therefore, be suspended, before complete paralysis of any part could occur from a cause which operates above the plexus. This may be experimentally shown on the Frog, whose crural plexus is formed by the interlacement of the component fasciculi of three trunks on each side; for section of the roots of one of these produces little effect on the general movements of the limb; and even when two are divided, there is no paralysis of any of its actions, all being weakened in nearly an equal degree. It is probable, however, that (as suggested by Dr. Gull) the chief use of this arrangement is to bring groups of muscles into relation with the different segments of the cord, in such a mode that their actions may be combined. and harmonized. We shall hereafter (CHAP. XII.) find reason to believe that the will does not at once act through the nerves upon the muscles; but that it plays (so to speak) upon the spinal cord, each segment of which has its own particular endowments, and ministers to a particular set of movements. And thus, the greater the variety of movements which any part is destined to perform, the more complicated will be the nervous plexus by which its muscles are connected with the centres of

motion.

378. The second primary element of the Nervous System, without which the fibrous portion would seem to be totally inoperative, is composed of nucleated cells, containing a finely granular substance, and lying somewhat loosely in the midst of a minute plexus of blood-vessels (Fig. 69, A). Their normal form may be regarded as globular (hence they have been termed nerve- or ganglion-globules); but this is liable to alteration from the compression they suffer, so that they may become oval or polygonal. The most remarkable change of form, however, which they undergo, is by an extension into one or more long processes, giving them a caudate or a stellate aspect (B, B). These processes are composed of a finely-granular substance, resembling that of the interior of the vesicle, with which they seem to be distinctly continuous; and if traced to a distance, they are frequently found to become continuous with the axis-cylinders of the nerve-tubes. As a general rule, according to Professor Kölliker, only one nerve-tube is connected with each ganglion-cell; but this rule is not without its exceptions, especially among the lower animals. The other processes seem to inosculate with those of other cells, so as to establish a direct communication between

them. In some instances, again, the ganglionic cell comes into relation with the fibre, not by sending out a prolongation which is continuous with it, but by being received into the cavity of the fibre (so to speak), by an extension of the tubular wall over it.. In other cases, moreover, the fibres merely pass around and amongst the ganglionic cells, without coming into direct connexion with them; and when this is the case, the ganglionic cells retain their simply globular character.-Besides the finely granular substance just mentioned, these cells usually contain a collection of pigment granules, which give them a reddish or yellowishbrown color: this, however, is frequently absent, especially among the lower animals.-The size of the vesicles is liable to great variation; the globular ones are usually between 1-300th and 1-1250th of an inch in diameter.

Primitive fibres and ganglionic globules of human brain, after Purkinje. A, ganglionic globules lying amongst varicose nerve-tubes, and blood-vessels, in substance of optic thalamus; a, globule more enlarged; b, small vascular trunk. B, B, globules with variously-formed peduncles, from dark portion of crus cerebri. 350 Diam.

379. The vesicles just described are aggregated together in masses of variable size; and are in some degree held together by the plexus of blood-vessels (Fig. 70), in the midst of which they lie. They are sometimes imbedded in a soft granular substance, which adheres closely to their exterior and to their processes; this is the case in the outer part of the cortical substance of the human brain. In other instances, each cell is enclosed in a distinct envelope, composed of smaller cells, closely adherent to each other and to the contained cell; such an arrangement is common in the smaller ganglia, and in the inner portion of the cortical substance of the brain. The substance which is made up of these peculiar cells, of the plexus of the blood-vessels in which they lie, and of the granular matter that is disposed amongst them, is altogether commonly known as the cineritious or gray substance; being distinguished by its color in Man and the higher animals at least, from the white substance (composed of nerve-tubes) of which the trunks of the nerves, as well as a large part of the brain and spinal cord, are made up. But this distinction is by no means constant; for the gray color, which is partly

due to the pigment-granules of the cells, and partly to the redness of the blood in the vessels, is wanting in the Invertebrata generally, and is not characteristically seen in the classes of Fishes and Reptiles. Moreover, when the ganglionic substance exists in small amount, even in

Man, its color is not sufficiently intense to serve to distinguish it; and, as we have already seen, there are nerve-fibres which possess a grayish hue. The real distinction evidently lies in the form of the ultimate structure, which is fibrous in the one case, and cellular or vesicular in the other; and these terms will be henceforth used to characterize the two kinds of nervous tissue, which have been now described.

380. A ganglion, then, essentially consists of a collection of nervevesicles or ganglion-globules, interspersed among the nerve-fibres; and it is in the presence of the former that it differs from a plexus, which it frequently resembles in the arrangement of the latter. When a nerve enters a ganglion, its component fibres separate and pass through the ganglion in different directions, so as to be variously distributed among the branches which pass out of it (Fig. 71); coming, in their

Dorsal ganglion of Sympathetic nerve of Mouse;-a, b, cords of connexion with adjacent sympathetic ganglia; c, c, c, c, branches to the viscera and spinal nerve; d, ganglionic globules or cells; e, nervous fibres crossing the ganglion.

course into relation with the vascular matter, which occupies the interior of the ganglion, in one or more of the modes already specified (§ 378). Some of the fibres may terminate in the cells of the ganglion,

and new ones may originate in them; so that there is no constant correspondence between the number of fibres which enter a ganglion, and of those who pass out of it.-The only exception to the general fact, that the vesicular matter occupies the centre of the ganglia, occurs in the brain of Vertebrata, in which it is chiefly disposed on the exterior, forming the cortical envelope. The reason for this variation is probably to be found in the very large amount of this substance, which the brain of the higher Vertebrata contains; and in the necessity of the free access of blood-vessels to it, which is provided for by a great extension of its surface beneath the investing vascular membrane (pia mater), more readily than it could be in any other mode.

381. But the vesicular matter is not found in the central masses only of the Nervous system; for it presents itself also at those parts of the surface or periphery which are peculiarly destined to receive the impressions that are to be conveyed to the central organs. Thus on the expansion of the optic-nerve which forms the retina, there is a distinct layer of ganglionic corpuscles or nerve-cells, with a minute plexus of vessels, possessing all the essential characters of the vesicular substance of the brain; and something of the same kind has been seen in connexion with the corresponding expansion of the olfactive and auditory nerves. Moreover, the study of the history of the development of these organs has shown that the vesicular matter of the retina is an offshoot (so to speak) from that of the optic ganglion, that of the labyrinth of the ear being in like manner an offshoot from that of the auditory ganglion. Thus it is obvious that the fibres of the connecting nerve are interposed between the cells of the peripheral and those of the central organs, for the sake of preserving that connexion between them which would otherwise have been interrupted; and that the vesicular matter is the active agent in the origination of those changes which take place as a consequence of sensory impressions, whilst for the conduction of such changes, the fibrous structure is alone required.

382. The ultimate distribution of the nerve-fibres in the skin and tongue, however, has not been so clearly made out, nor can their relation to cells be distinctly traced. These fibres are distributed, for the most part, to the papillæ, in which they can be frequently seen to form Fig. 72. Fig. 73.

loops (Fig. 72), accompanied by similar loops of blood-vessels (Fig. 73): but no such loops can be seen in the fungiform papillæ of the tongue,