NERVOUS CENTRE OF MUSCULAR CONTRACTION. 387 



which excites them to action ; and accurately regulates the extent and 

 velocity of muscular contraction. Yet volition is not the sole excitant 

 of such contraction. If we irritate any part of the encephalon or 

 spinal marrow, or any of the nerves proceeding from them, muscular 

 movements are excited; but they are not regulated as when under the 

 influence of volition. The whole class of involuntary motions, or rather 

 of those executed by the muscular system of organic life, is of this 

 kind, including the action of many of the most important organs, 

 heart, intestines, blood-vessels, &c. The involuntary muscles equally 

 require a stimulus to excite them into action ; but, as their name 

 imports, they are removed from the influence of volition. In certain 

 diseased conditions, we find, that all the voluntary muscles assume in- 

 voluntary motions ; but this is owing to the ordinary volition being 

 interfered with, and to some direct or indirect stimulation affecting the 

 parts of the cerebro-spinal axis concerned in muscular contraction; or, 

 if the effect be local, to some stimulation of the nerve proceeding from 

 the axis to the part. Of this kind of general involuntary contraction 

 of voluntary muscles, we have a common example in the convulsions of 

 children ; and one of the partial kind, in cramp or spasm. 



The will, then, is the great but not the sole regulator of the supply 

 of voluntary nervous influence. This is confirmed by experiment. If 

 a portion of the spinal marrow be divided, so as to separate it from all 

 communication with the encephalon, the muscles cannot be affected by 

 the will ; but they contract on irritating the part of the spinal marrow, 

 from which its nerves proceed. It has, hence, been presumed by some 

 physiologists, that volition is only the exciting and regulating cause of 

 the nervous influx; and that the latter is the immediate agent in pro- 

 ducing contraction; and they affirm, that as, in the sensations, the im- 

 pression is made on the nerve, and perception effected in the brain, so, 

 in muscular motion, volition is the act of the encephalon, and the nerv- 

 ous influx to a part corresponds to the act of impression. 



With regard to the seat of this nervous centre of muscular contrac- 

 tion, much discrepancy has existed amongst modern physiologists. It 

 manifestly is not in the whole encephalon, as certain portions of it may 

 be irritated in the living animal without exciting convulsions. Parts 

 of it, again, may be removed without preventing the remainder from 

 exciting muscular contraction when irritated. In the experiments of 

 M. Flourens, the cerebral lobes were taken away, yet the animals, when 

 stimulated, were susceptible of motion; and, whenever the medulla 

 oblongata was irritated, convulsions were produced. Its seat is not, 

 therefore, in the whole encephalon. M. Rolando refers it to the cere- 

 bellum. He asserts, that on removing the cerebellum of living animals, 

 without implicating any other part of the encephalon, they preserved 

 their sensibility and consciousness, but were deprived of the power of 

 motion. This occurred to a greater extent in proportion to the severity 

 of the injury inflicted on the cerebellum. If the injury was slight, the 

 loss of power was slight; and conversely. Impressed with the resem- 

 blance between the cerebellum of birds and the galvanic apparatus of 

 the torpedo ; and taking into consideration the lamellated structure of 

 the cerebellum, which, according to him, resembles a voltaic pile; and 



