cxlvi NERVOUS SYSTEM. 



tion, which are not unusual, both nerves commonly give and receive fibres ; 

 so that, after the junction, each contains a mixture of fibres derived from 

 two originally distinct sources. More rarely the fibres pass only from one 

 of the nerves to the other, and the contribution is not reciprocal. In the 

 former case the communicating branch or branches will of course contain 

 fibres of both nerves, in the latter of one only. 



In other cases the branches of a nerve, or branches derived from two or 

 from several different nerves, are connected in a more complicated manner, 

 and form what is termed a plexus. In plexuses of which the one named 

 " brachial " or " axillary," formed by the great nerves of the arm, and the 

 " lumbar" and "sacral," formed by those of the lower limb and pelvis, are 

 appropriate examples the nerves or their branches join and divide again 

 and again, interchanging and intermixing their fibres so thoroughly that, 

 by the time a branch leaves the plexus, it may contain fibres from all the 

 nerves entering the plexus. Still, as in the more simple communications 

 already spoken of, the fibres, so far as is known, remain individually distinct 

 throughout. 



Some farther circumstances remain to be noticed as to the course of the fibres in 

 nerves and nervous plexuses. 



Gerber has described and figured nerve-fibres, which, after running a certain way 

 in a nerve, apparently join in form of loops with neighbouring fibres of the same 

 bundle, and proceed no further. Such loops might of course be represented as 

 formed by fibres which bend back and return to the nervous centre ; and so Gerber 

 considers them. He regards them as looped terminations of sentient fibres appro- 

 priated to the nerve itself as the nervi nerrorum, in short, on which depends the 

 sensibility of the nerve to impressions, painful or otherwise, applied to it elsewhere 

 than at its extremities. The whole matter is, however, involved in doubt : for, 

 admitting the existence of the loops referred to, which yet requires confirmation, it 

 is not impossible that they may be produced by fibres which run back only a certain 

 way, and then, entering another bundle, proceed onwards to the termination of the 

 nerve. Again, it has been supposed, that, in some instances of nervous conjunc- 

 tions, certain collections of fibres, after passing from one nerve to another, take a 

 retrograde course in that second nerve, and, in place of being distributed periphe- 

 rally with its branches, turn back to its root and rejoin the cerebro-spinal centre. 

 An apparent example of such nervous arches without peripheral distribution is 

 afforded by the optic nerves, in which various anatomists admit the existence of 

 arched fibres that seem to pass across the commissure between these nerves from 

 one optic tract to the other, and to return again to the brain. These, however, are 

 perhaps to be compared with the commissural fibres of the brain itself, of which 

 there is a great system connecting the symmetrical halves of that organ. But 

 instances of a similar kind occurring in other nerves have been pointed out by 

 Volkmann ; as in the connection between the second and third cervical nerves of 

 the cat, also in that of the fourth cranial nerve with the first branch of the fifth in 

 other quadrupeds, and in the communications of the cervical nerves with the spinal 

 accessory and the descendens noni. But certain fibres of the optic nerves take a 

 course deviating still more from that followed generally, for they appear to be con- 

 tinued across the commissure from the eyeball and optic nerve of one side to the 

 opposite nerve and eye, without being connected with the brain at all, and thus to 

 form arches with peripheral terminations, but no central connection. In looking, 

 however, for an explanation of this arrangement, it must be borne in mind that the 

 retina contains nerve-cells, like those of the nervous centres, and perhaps the fibres 

 referred to may be intended merely to bring the collections of nerve-cells of the two 

 sides into relation independently of the brain. Julius Arnold has found an arrange- 

 ment of fibres at the junctions of the nerve-plexus of the iris similar to that in the 

 optic commissure.* 



The disposition of the fibres at the points of division and junction of the branches 



* Virchow's Arch. 1863. 



