143 NEEYOUS TISSUE. 



ment of the fibres in tlie«e cases, it is to be observed, that, in the 

 Ijranching of a nerve, collections of its fibres successively leave the trunk 

 and form branches ; and that, Avhcn different nerves or their branches 

 intercommunicate, fibres pass from one nerve to become associated with 

 those of the other in their further progress ; but in neither case (unless 

 towards their ])eripheral terminations) is tliere any such thing as a 

 division or splitting of an elementary nerve-fibre into two, or an actual 

 junction or coalescence of two such fibres together. 



A communication between two nerves is sometimes effected by one 

 or two connecting branches. In such comparatively simple modes of 

 connection, whicli 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 contribu- 

 tion is not reciprocal. 



In other cases the branches of a nerve, or branches derived fi'ora two 

 or from several diffbrent 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. 



In some instances of nervous conjunctions 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 peripherally -with its branches, turn back to its 

 root and rejoin the cerebro-spinal centre. An ajiparent 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 

 commissiiral 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 be- 

 tween 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 quadmpeds, and in the 

 communications of the cer^^cal nerves with the spinal accessory and the descen- 

 dens noni. But certain fil^res of the optic nerves take a coiurse deviating still 

 more from that followed generally, for they appear to be continued 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 

 is itself originally an outgrowth from the brain and 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 inde- 

 pendently of the brain. Julius Arnold has found an arrangement 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 



* Yirchow's Arch, 18G3. 



