572 FUNCTIONS OF THE CEREBRO-SPINAL NERVOUS SYSTEM. 



terminates or forms a loop at the other; in the terminal ramifications of the 

 nerves, however, a subdivision and plexif'orm arrangement of the fibres is 

 frequently observable. Although the fasciculi occasionally intermix and 

 exchange fibres with one another (as occurs in a plexus), the fibres them- 

 selves never inosculate. Each fibre would seem, therefore, to have its ap- 

 propriate office, which it cannot share with another. Several objects appear 

 to be attained by the plexiform arrangement. In some instances it serves 

 to intermix fibres, which have endowments fundamentally different; for 

 example, the Spinal Accessory nerve, at its origin, appears to be exclusively 

 motor, and the roots of the Pneumogastric to be exclusively afferent; but 

 by the early admixture of these, a large number of motor fibres are imparted 

 to the Pneumogastric, and are distributed in variable proportion, with its 

 different branches; whilst a few of its sensory filaments seem to enter the 

 Spinal Accessory. In other instances, the object of a plexus 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 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 several seg- 

 ments 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 experi- 

 ment; 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. 1 All consideration 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 ; 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 dif- 

 ferent 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 pre- 

 sent themselves among the higher Invertebrata. 



463. The following statements, in which the doctrines of Prof. Mailer 2 are 

 adopted with some modifications and additions, embody the general princi- 

 ples 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 regard to the isolated character of each fibril, and the identity of its en- 

 dowments through its whole course. 



1 Striking illustrations of t bo importune* 1 of the plexiform communication of norvos 

 are afforded by such cases as that recorded by Mr. Savory (Lancet Any 1st, 18'i8), 

 wbore, after excision of a portion of tbo. musculo spiral nerve, tbe sensibility of tlie 

 part, supplied bv it, tbouirb impaired, was still to some extent retained, in consequence 

 probably of the communications Hint exist between i lie radial and external cutaneous 

 nerves in the forearm. See also W Gull, Gulstonian Lectures on the Nervous Sys- 

 tem, in Medical Times 1849, p. 3 7 '2. 



2 Elements: of Physiology, translated by Dr. I3aly ; pp. 68">, 680. 



