ELEMENTARY FUNCTIONS OF THE NERVOUS STRUCTURE. 95 



kind of plexus, which evidently contributes to the very powerful and harmo- 

 nious movements of the arms of this Cephalopoda It is considered by Dr. Ali- 

 son, that the origin of the trunks which supply the various muscles of the ex- 

 tremities from several segments of the spinal cord, instead of one, has the further 

 use of enabling the mind to vary, in greater degree than would otherwise be 

 possible, the power with which the muscle shall be called into action; and this 

 idea is certainly supported by the curious fact, that it is in the nerves of the 

 extremities only that this plexiform arrangement prevails ; and that the nerves 

 of the eyeball, in whose action there is an equal degree of consentaneousness, 

 but far less variety of power, arise from single points of the cerebro-spinal 

 axis. It is further considered by Dr. A., that the plexiform arrangement may 

 enable the sensations proceeding from the muscles (which are important guides 

 in their movement), to be more distinct, and consequently more easily discrimi- 

 nated from one another, than they would otherwise be ; and there does not 

 seem any reason why the same view should not be extended to the sensory 

 impressions communicated from the general surface of the extremities. 



116. There is no valid reason to believe that the nervous fibres undergo 

 any change of function along their whole course ; and we may, probably, com- 

 pare them, by way of analogy only, to the conducting wires of a galvanic 

 apparatus. But it is evident that the special structure into which they pass, 

 the plexus of blood-vessels and ganglionic globules, must have some particu- 

 lar function ; and there seems no reasonable ground for doubt, that in this struc- 

 ture, those changes originate which are conducted by the fibres to distant 

 points. Following out the same analogy, then, we might compare this struc- 

 ture with the galvanic combination, by which the electric influence is gene- 

 rated, that is conveyed to some distant point by the connecting wires and 

 there produces a decomposition or other similar change; and there seems 

 to be as much reason for thus assigning the functions of the production and 

 conduction of nervous power to the gray and white portions of the nervous 

 system respectively, as there is for attributing the production of electric power 

 to the galvanic trough, and its conduction to the connecting wires. Wherever 

 we have reason to believe that new power is generated, there do we find gray 

 matter ; and, on the other hand, there are few, if any, instances in which gray 

 matter is present, without our being able to assign to it some obvious purpose 

 of this kind. 



117. The belief that all changes in the nervous system, whether they take 

 place at the centre or at the periphery, originate at the points in which the 

 fibres come into relation with the vascular plexus, derives confirmation from 

 the well-known dependence of these changes upon the activity of the circu- 

 lation through the part at which they occur. Thus, if the circulation of blood 

 through the brain be suspended for an instant, insensibility supervenes. If 

 the cause of suspension be local only, the remdnder of the nervous system 

 may still be excited to action. This was the case in experiments made by Sir 

 A. Cooper. After having tied both carotid arteries in a dog, he compressed 

 the vertebral trunks, and immediate insensibility resulted, proving the inactive 

 condition of the brain ; whilst convulsions also occurred, showing that the 

 functions of the spinal cord were not suspended, but only deranged. But if, 

 as in syncope, the circulation through the spinal cord also be weakened, its 

 power of producing motions in respondence to impressions is diminished in 

 like proportion. In the same manner the production of impressions on the 

 peripheral origins of the afferent nerves, appears equally dependent upon the 

 active influence of the vascular system. Every one knows that cold, which 

 retards the circulation of blood through, the skin, diminishes also its sensi- 

 bility ; and obstruction to the circulation by any other cause, such as pressure 

 on the arterial trunks, produces the same effect. Moreover, it is always 



