PLAN OF THE CEREBRAL STRUCTURE.

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slender core of grey substance;-if we trace the fibres of the cord upwards, we find them continuing into the medulla oblongata, the first and lowest portion of the brain. Of the whole mass of fibres entering the medulla oblongata, the larger portion pass up into the cerebellum and the pons Varolii; while a part terminates in the grey substance of the medulla itself; and from that grey substance other fibres take their rise and proceed onwards, in the company of the throughgoing fibres of the cord. Thus the emerging white matter of the medulla oblongata is partly the fibres that entered it, as a continuation of the cord, and partly the fibres originating in the grey central matter of the medulla, replacing as it would seem, those that terminated there. From the pons Varolii, where we come next, the white fibres advance in various directions; intersecting with transverse fibres connecting the two halves of the cerebellum, and passing upwards towards the cerebrum proper. The fibres thus going upwards constitute the crura, peduncles, or stems of the cerebrum, and seem destined to terminate in the grey matter of the convoluted surface of the hemispheres. But in passing through the ganglia of the brain-the thalami optici and corpora striata— the arrangement described above is repeated; that is to say, while a large part of the fibres pass clear through the ganglionic masses, the rest stop short in the grey substance of those masses, which grey substance gives origin to other fibres to pass out with those that had an uninterrupted course through the bodies alluded to. Both sets together—those passing through, and those originating in, the grey substance of the corpora striata, or thalami optici-constitute a portion of the white or fibrous substance of the hemispheres, spreading out and terminating in the grey matter, or cortical layer, of the convolutions. They are the first of three classes of fibres, described above, as constituting the white matter of the cerebrum; that is to say, the ascending or diverging class.

Whatever number of central masses we may calculate as interposed between the spinal cord beneath and the convoluted surface of the cerebrum, the manner of communication between

them is found to be as now stated. The fibres passing between one intermediate mass and another are partly transmitted and partly arrested. Wherever grey matter exists, there is the commencement or termination of white matter. The fibres that enter the cerebellum from the medulla oblongata, terminate in whole, or in part, in its outer layer of grey substance, and in that substance a new set of fibres originate to pass to other parts of the brain, as the corpora quadrigemina, the hemispheres, &c., and from one half of the cerebellum to the other. The fibres spreading out, as already mentioned, in the hemispheres towards the convoluted grey surface will have had very various origins. Some have perhaps come all the way from the extremities of the body, passing by the spinal cord, medulla oblongata, cerebellum, pons Varolii, thalami optici, &c.; others have originated in the grey matter of the cord, passing without a break through all the intervening centres; a third class may have had their rise in the grey matter of the medulla oblongata; a fourth in the grey matter of the pons; a fifth in the cerebellum; a sixth in the corpora quadrigemina; others in the thalami optici or corpora striata ; besides other more minute sources.

The arrangement may thus be seen to resemble the course of a railway train. The various central masses are like so many stations where the train drops a certain number of passengers and takes up others in their stead, whilst some are carried through to the final terminus. A system of telegraph wires might be formed to represent exactly what takes place in the brain. If from a general terminus in London a mass of wires were carried out to proceed towards Liverpool, and if one wire of the mass were to end at each station, while from the same station new wires arose, one for every station further on, a complete and perfectly independent connexion could be kept up between any two stations along the line. Calling the stations a, b, c, d, e; there would be from a the London terminus, the wires, ab, ac, ad, ae; from b, would arise, bc, bd, be; from c, cd, ce; and from d, de. The mass of wires found on the road at a point between c and d, would be a e, or the one through-going wire, be and bd, ce and cd; five wires in all,

CEREBRO-SPINAL NERVES.

which would be the number sustained throughout.

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This

system of telegraph communication would be, so far as appears, the type of nervous communication among the various masses strung together in the cerebro-spinal axis or centre.

But it is only a very small number of the fibres ending in the convolutions of the hemisphere that have performed the entire course from the extremities of the body. Indeed, some Anatomists have doubted the existence of any such uninterrupted fibres, or at least consider that they have not been. traced with unimpeachable certainty through the entire line of centres. Others, however, whom we are compelled to acknowledge as among the highest authorities in Anatomy maintain that such fibres have been undoubtedly traced.

The application of this view of the plan of structure of the brain will appear in the sequel, after we have ascertained the distinctive functions or uses of the two kinds of nervous matter.

OF THE CEREBRO-SPINAL NERVES.

9. By the cerebro-spinal nerves are meant the connexions of the cerebro-spinal centre with the different parts of the body. These connexions consist of ramifications of nerve cords, threads, or bundles, arising in the central masses, and distributed like the blood vessels, by subdividing and spreading themselves over the various organs and tissues, thereby establishing a connection between the brain and the remotest extremities.

'These nerves are formed of the nerve fibres already described, collected together and bound up in membranous sheaths. A larger or smaller number of fibres inclosed in a tubular sheath form a small round cord, usually named a funiculus; if a nerve be very small, it may consist of but one such cord, but in larger nerves several funiculi are united together into one or more larger bundles, which, being wrapped up in a common membranous covering, constitute the nerve (Fig. 5.) Accordingly, in dissecting a nerve, we first come to an outward covering, formed of cellular tissue, but often so strong and dense, that it might well be called fibrous. From

this common sheath we trace lamina passing inwards, between the larger and smaller bundles of funiculi, and finally between

FIG. 5.*
A

B

the funiculi themselves, connecting them together as well as conducting and supporting the fine blood vessels which are distributed to the nerve.'

'The funiculi of a nerve are not all of one size, but all are sufficiently large to be readily seen with the naked eye, and easily dissected out from each other. In a nerve so dissected into its component fasciculi, it is seen that these do not run along the nerve as parallel insulated cords, but join together obliquely at short distances as 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 what we find to hold in a plexus formed by the branches of different nerves. It must be distinctly understood, however, that in these communications the proper 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 funiculi and undergo still further intermixture. But through all these successive associations, the nerve fibres remain, as far as known, individually distinct, like interlaced threads in a rope.'

"Vessels.-The blood vessels of a nerve supported by the nerve sheath divide into very fine capillaries, said by Henlé

Represents a nerve consisting of many smaller cords or funiculi, wrapped up in a common cellular sheath. A, the nerve. B, a single funiculus drawn out from the rest (after Sir C. Bell).'-QUAIN, p. ccix.

BRANCHING AND ORIGINS OF NERVES.

1

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to measure in the empty state not more than oth of an inch in diameter. These, which are numerous, run parallel with the funiculi, but are connected at intervals by short transverse branches, so as in fact to form a network with very long narrow meshes.

'Branching and Conjunction of Nerves.-Nerves in their progress very commonly divide into branches, and the branches of different nerves not unfrequently join with each other. As regards the arrangement of the fibres in these cases, it is to be observed, that, in the branching of a nerve, portions of its fibres successively leave the trunk and form branches; and that when different nerves or their branches intercommunicate, fibres pass from one nerve and become associated with those of the other in their further progress; but in neither case (unless at their peripheral terminations) is there any such thing as a division or splitting of an elementary nerve into two, or an actual junction or coalescence of two such fibres together.'-SHARPEY; QUAIN, Introduction, ccix-xii.

'Origins or Roots of the Nerves.-The cerebro-spinal nerves, as already said, are connected by one extremity to the brain or to the spinal cord, and this central extremity of a nerve is, in the language of anatomy, named its origin or root. In some cases the root is single, that is, the funiculi or fibres by which the nerve arises are all attached at one spot, or along one line or tract; in other nerves, on the contrary, they form two or more separate collections, which arise apart from each other, and are connected with different parts of the nervous centre, and such nerves are accordingly said to have two origins or roots. In the latter case, moreover, the different roots of a nerve may differ not only in their anatomical characters and connexions, but also in function, as is well exemplified in the spinal nerves, each of which arises by two roots, -an anterior and a posterior; the former containing the motory nerves of the fibre, the latter the sensory.

'The fibres of a nerve, or at least a considerable share of them, may be traced to some depth in the substance of the brain or spinal cord, and hence the term 'apparent or superficial origin', has been employed to denote the place where

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