652 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



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

 ing endowments. Thus the brachial plexus mixes together the fibres arising from 

 five segments of 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 

 connection with the nervous centres, they would have been much more liable to 

 paralysis than at present. By means of the plexus, every part is supplied with 

 fibres arising from each segment of the spinal cord ; and the functions of the 

 whole must therefore be suspended, before complete paralysis of any part can 

 occur from a cause which operates above the plexus. Such a view is borne out 

 by direct experiment ; for it has been ascertained by Panizza that in Frogs, 

 whose crural plexus is much less complicated than that of Mammalia, section 

 of the roots of one of the three nerves which enter into it produces little 

 effect on the general movements of the limb; and that, even when two are 

 divided, there is no paralysis of any of its actions, all being weakened in a 

 nearly similar degree. But, as Dr. Gull has pointed out, 1 one use of such a 

 plexus as the brachial or the crural appears to be, to bring the muscles which 

 derive their nervous supply from it, into relation with different ganglionic seg- 

 ments of the Spinal Cord ; each of which may exert a diverse action, either in 

 virtue of its own endowments, or of the influence of the will upon it ; so that 

 groups of muscles may thus be associated for combined actions. All considera- 

 tion of the mode in which we make use of our muscles, and of the power 

 which we have over them, leads to the conclusion that each ganglionic centre 

 has a specific and limited sphere of influence, producing certain movements and 

 no others j hence for the execution of a variety of movements in harmonious 

 combination with each other, it seems requisite that the nervous supply of each 

 muscle should be derived from several different centres ; and thus it is that the 

 complication of plexuses comes to be related to the variety of movements of 

 the parts supplied through them. It is not a little interesting to remark that 

 arrangements of a similar kind should present themselves among the higher 

 Invertebrata. Thus, in Hymenopterous Insects (as first pointed out by Mr. 

 Newport), there is a plexiform interlacement between the nerves of the anterior 

 and of the posterior pairs of wings, which act very powerfully together j whilst 

 in the Coleoptera, in which the anterior wings are converted into elytra, and 

 are motionless during flight, the nerves supplying each pair run their course 

 distinctly. In the Octopus, or Poulp, again, the trunks which radiate from the 

 cephalic mass to the eight large arms surrounding the head are connected by a 

 circular band, forming a kind of plexus, which seems to contribute to the very 

 powerful and harmonious movements of the arms of this Cephalopod. 



689. The following statements, in which the doctrines of Prof. Miiller 3 are 

 adopted with some modifications and additions, embody the general principles 

 ascertained by experiment, respecting the transmission of sensory and motor 

 impressions along the nerves which respectively minister to them. Their 

 rationale will be at once understood, from the facts already mentioned in re- 

 gard to the isolated character of each fibril, and the identity of its endowments 

 through its whole course. 



I. When the whole trunk of a sensory nerve is irritated, a sensation is pro- 

 duced, which is referred by the mind to the parts to which its branches are 

 ultimately distributed ; and if only part of the trunk be irritated, the sensation 

 will be referred to those parts only which are supplied by the fibrils it contains. 

 This is evidently caused by the production of a change in the sensorium, cor- 

 responding with that which would have been transmitted from the peripheral 



1 "Gulstonian Lectures on the Nervous System," in "Medical Times," 1849, p. 372. 



2 "Elements of Physiology," translated by Dr. Baly ; pp. G80, 080. 



