FUNCTIONS OR THE NERVOUS SYSTEM. 



the nervous centres ; whilst the other conveys the influence of these central 

 organs to the structure at large, and especially to the muscular system. Hence 

 it will be convenient to denominate the first afferent fibres, and the second effe- 

 rent. These are to be regarded as general terms, expressing only the direc- 

 tion in which they propagate the changes to which they are subservient. The 

 nature of these changes will be a subject of future inquiry. Although, as just 

 stated, the fibres can no where be said to have free extremities, yet the afferent 

 fibres, which convey to the central organs the influence of changes taking 

 place at the periphery, may be said to originate in the ganglionic matter dif- 

 fused through the latter, and to terminate in the central ganglia: whilst the 

 efferent fibres, which convey to the muscles the motor influence generated in 

 these last organs, may be said to originate in the central ganglia, and to termi- 

 nate in the muscular tissue. 



114. Every fibre, there is reason to believe, runs a distinct course, between 

 the central organ, in which it loses itself at one extremity, and the muscle or 

 organ of sense in which it terminates at the other. Each Nervous Trunk is 

 made up of several fasciculi of these fibres ; and each fasciculus is composed 

 of a large number of the ultimate fibres themselves. Although the fasciculi occa- 

 sionally intermix and exchange fibres with one another, (as occurs in what is 

 termed a plexus,) the fibres themselves never inosculate. Each fibre would 

 seem, therefore, to have its appropriate office, which it cannot share with ano- 

 ther. The objects of a plexus are twofold. 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 par vagum are as exclusively sensory ; but by the early admix- 

 ture of these, a large number of motor fibres are imparted to the par vagum, 

 and are distributed, in variable proportion, with its different branches ; whilst 

 few of its sensory filaments seem to enter the spinal accessory. In other in- 

 stances the object of a plexus appears to be, to give a more advantageous dis- 

 tribution to fibres, which all possess corresponding 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 Fanizza 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. 



115. It is not unlikely also that, by this arrangement, a consent aneousness 

 of action is in some degree favoured, as is supposed by Sir C. Bell; for com- 

 parative anatomy shows that something resembling it may be traced, wherever 

 a similar purpose has to be attained. "Thus, in the Hymenoptera, there is a 

 similar interlacement between the nerves of the anterior and posterior pairs of 

 wings, which act very powerfully together ; 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 anus surrounding the head, are connected by a circular band ; forming a 



