NERVOUS SYSTEM. (NERVE.) 



595 



plexus, and so returning to the spinal cord. 

 The nervous arch which is thus formed must 

 evidently establish a communication between 

 the cervical region of the spinal cord and that 

 portion of the medulla oblongata whence the 

 ninth nerve appears to arise. 



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 brain. 



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 fibresof 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 anastomosis 

 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, passed 

 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 neunlemma. Certain primitive 

 fibres emerge from and return to the nervous 

 centre, forming a loop, with convexity directed 

 towards the periphery, without connecting 

 themselves with any peripheral texture or going 

 beyond the nerve-sheath. Gerber has desig- 



* Miillei-'s Archiv. 



nated these loops nervi nervorum, from a sup- 

 posed rather fanciful analogy to the vasavaaorum. 



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 

 axillae, the loins, and the sacral 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 from 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 

 Jig. 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 mutter. 

 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 demonstrated. 

 We shall recur to this interesting and important 

 question in the article NERVOUS CENTRES. 

 Termination of nerves. Under this terra 



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