LAWS OF NERVOUS TRANSMISSION. 241 



represent the arrangement of the particles, in the condition of equilibrium or quiescence, and 

 this condition be disturbed at one extremity, by the operation of a new attraction upon the 

 first particle a, a new arrangement will instantaneously take place throughout: this may be 

 represented by 



a ba ba ba ba ba ba ba b 



which shows b in a free state at the opposite end, ready to exert its influence upon anything 

 submitted to it. It is probable that in this respect there is an analogy between the Nervous 

 and electrical forces ; and that, instead of speaking of the "transmission of nervous influence" 

 along a nerve, we should describe the change as the production of a "polar state" in the 

 nervous trunk; as first pointed out by Messrs. Todd and Bowman (Physiological Anatomy, 

 vol. i. p. 240). 



298. 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 

 occasionally 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 

 another. 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 afferent; but by 

 the early admixture 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 Acces- 

 sory. In other instances, the object of a plexus appears to be, to give a more 

 advantageous distribution to fibres, which all possess corresponding endow- 

 ments. 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 seg- 

 ment 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. It is not unlikely also that, by 

 this arrangement, a consentaneousness of action is in some degree favoured, as 

 is supposed by Sir C. Bell; for comparative 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 to- 

 gether; 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. 



299. The following statements, in which the language of Mtiller is adopted 

 21 



