We find in connexion with the optic nerve a remarkable example of this kind of anastomosis, which, as in the instance just mentioned, serves more to connect different portions of the ner vous centre than to associate particular nerves. In the optic tracts of man three series of fibres may be distinguished, one which passes to the retina of the same side, another series which goes to the retina of the opposite side, decus sating with the corresponding fibres from that side, and a third which passes from right to left, being apparently identified with or 'fused into one another at what is called the commissure, and forming a series of nervous arches, which serve to connect the opposite sides of the brain. These arches are convex towards the eyes and concave towards the brain. In the mole, in which I have failed to discover an optic nerve, this commissural band exists alone, the other two series of fibres being absent. Mr. Mayo has given a representation of these three sets of fibres belonging to the human chiasma in his admirable plates of the bmin.
Volkmann gives an account of several anas tomoses of this kind which he distinguishes by. the expression " verschmelzungen," to which that of " fusion " appears sufficiently to correspond. The fibres of one nerve appear as if they had been fused into those of an adjacent one, and thus return to some part of the cerebro-spinal centre different from that at which it had emerged. The instances cited by Volkmann are as follows : 1. In the calf he has found an anastornosis between the fourth pair of nerves and the first branch of the fifth pair, forming an arch from the convexity of which several branches passed off in a peripheral direction. By far the greater part of these appeared, on microscopic exami nation, to receive their fibres from the fourth ; while those fibres of the fifth which contributed to the formation of the nervous arch, pa.ssed centripetally to the brain, bound up in the sheath of the fourth nerve. 2. A similar ner vous arch is found very generally among mammifera between the second or third cer vical nerve and the accessory. Certain fibres, when traced from the former nerve, appeared to pass to the centre in the sheath of the latter. This anastomosis Volkmann found in the human subject, and in horses, dogs, calves, and cats.* Another example of this kind of anastomosis has been described by Gerber, but I am not aware whether his statements have been con firmed by other observers. This consists of one or more simple loops contained in one and the same neurilemma. Certain primitive fibres emerge from and return to the nervous centre, forming a loop, with convexity directed towards the periphery, without connecting theniselves with any peripheral texture or going beyond the nerve-sheath. Gerber has desig nated these loops nervi nervorum, frkn a posed rather fanciful analogy to the vasavasorwn.
Plexuses.—The plexuses are nervous anasto moses of the most complicated and extensive kind. Those which are connected with the
spinal nerves are found in the neck, the axillx the loins, and the sacml region, and are well described by anatomists. There are also plexuses connected with the fifth nerve, the portio dura of the seventh, the glosso-pharyn geal, and the par vagum. Each plexus is formed by the breaking up of a certain number of nervous trunks, the subdivisions of which unite together to form secondary nerves, and these again, by further interchange of fibres, give rise to nerves which emerge from the plexus, and consequently in their construction may derive their fibres from several of the trunks that enter the plexus.
The object of the various kinds of anasto mosis of nerves above enumerated appears to be to associate together nervous fibres con nected with different parts of the brain or spinal cord. Thus nerve-tubes of different pro perties or endowments become united together in one sheath, forming compound nerves; and certain sets of muscles, instead of receiving their nerves from a very limited portion of the cerebro-spinal centre, are supplied ftom a con siderable extent of that centre, and each muscle may probably receive nerves which arise in dif ferent and distant parts of the spinal cord or brain, an arrangement whereby remote parts of those centres may be brought into connection with neighbouring muscles or other parts, or even with a single muscle.
Origin of nerves.—The connexion of a nerve with the nervous centre is called by descriptive anatomists its origin. The determination of the exact nature of this connexion is of the last importance to the adoption of a correct theory of nervous action. Yet but little is known upon this subject. The fibres of the nerves are continuous with some of those fibres of the centre, in passing into which they experience considerable diminution of size and perhaps some change of texture, (see fig. 330, A,) as evinced by their much greater tendency to become varicose under mecha nical means than we generally find in the nerves themselves. Thus far we may con fidently assert, that every nerve at its central extremity forms a connexion with grey niatter. This fact, proved by anatomy to be constant and universal, may be considered as a law of the morphology of nerve which has the most important bearing upon its physiological action. What is the precise nature of the connexion between the two kinds of nervous matter in the centres has not yet been determined. We can see the white nerve-tubes passing between the elements of the grey matter and the vascular plexus, in the meshes of which they are depo sited; but whether they form any continuity of substance with those elements, or simply come into contact with them,has yet to be d emonstrated. We shall recur to this interesting and important question in the article NERVOUS CENTRES.