I 



COMMUNICATIONS BETWEEN NERVOUS TRUNKS. 219 



the latter sends a smaller number of fibres into the former, these being 

 chiefly of the gelatinous kind. 



377. Sometimes we find the fasciculi of several distinct trunks united 

 into an extensive plexus; the sole object of which appears to be, to 

 give a more advantageous distribution to fibres, which all possess corre- 

 sponding endowments. Thus the brachial plexus mixes together the 

 fibres arising by five pairs of roots, on either side, from the spinal cord ; 

 and sends off five principal trunks to supply the arm. Now, if each of 

 these trunks had arisen by itself, from a distinct segment of the spinal 

 cord, so that the parts on which it is distributed had only a single con- 

 nexion with the nervous centres, they woulcl have been much more 

 liable to paralysis than they are. By means of the plexus, every part 

 is supplied with fibres arising from each of the five segments of the 

 spinal cord ; and the functions of the whole must, therefore, be sus- 

 pended, before complete paralysis of any part could occur from a cause 

 which operates above the plexus. This may be experimentally shown 

 on the Frog, whose crural plexus, is formed by the interlacement of the 

 component fasciculi of three trunks on each side ; for section of the 

 roots of one of these produces little effect on the general movements of 

 the limb ; and even when two are divided, there is no paralysis of any 

 of its actions, all being weakened in nearly an equal degree. It is pro- 

 bable, however, that (as suggested by Dr. Gull) the chief use of this 

 arrangement is to bring groups of muscles into relation with the different 

 segments of the cord, in such a mode that their actions may be combined 

 and harmonized. We shall hereafter (CHAP, xn.) find reason to believe 

 that the will does not at once act through the nerves upon the muscles ; 

 but that it plays (so to speak) upon the spinal cord, each segment of 

 which has its own particular endowments, and ministers to a particular 

 set of movements. And thus, the greater the variety of movements 

 which any part is destined to perform, the more complicated will be the 

 nervous plexus by which its muscles are connected with the centres of 

 motion. 



378. The second primary element of the Nervous System, without 

 which the fibrous portion would seem to be totally inoperative, is com- 

 posed of nucleated cells, containing a finely granular substance, and 

 lying somewhat loosely in the midst of a minute plexus of blood-vessels 

 (Fig. 69, A). Their normal form may be regarded as globular (hence 

 they have been termed nerve- or ganglion-globules) ; but this is liable to 

 alteration from the compression they suffer, so that they may become 

 oval or polygonal. The most remarkable change of form, however, 

 which they undergo, is by an extension into one or more long processes, 



'giving them a caudate or a stellate aspect (B, B). These processes are 

 composed of a finely-granular substance, resembling that ^of the interior 

 of the vesicle, with which they seem to be distinctly continuous ; and if 

 traced to a distance, they are frequently found to become continuous 

 with the axis-cylinders of the nerve-tubes. As a general rule, according 

 to Professor Kolliker, only one nerve-tube is connected with each 

 ganglion-cell; but this rule is not without its exceptions, especially 

 among the lower animals. The other processes seem to inosculate with 

 those of other cells, so as to establish a direct communication between 



