86 ANIMAL-SENTIENT FORCES. [i. 



hypothesis, and other learned persons have rejected it entirely, 

 on the ground that although impressions, whether external or 

 internal, may excite convulsions in muscles, they excite no 

 perceptible movement in the nerves. Nevertheless, as the 

 cerebral impression undoubtedly excites movement in the ter- 

 minal points of the nerves (147), this may be possible and pro- 

 bable, wherever else it is deflected from its direct course (151). 

 Further proofs on this point will be given subsequently (178). 



161. Amongst the mechanical machines, in which the 

 nerves are so distributed as to be completely incorporated with 

 them, the muscles hold the first place. Nerves penetrate all 

 muscles, being distributed together with the blood-vessels through 

 the cellular tissue, losing, however, their firm coat, and be- 

 coming soft, before they become so minute as to be no longer 

 traceable (^ Haller^s Physiol.,^ § 398). It maybe also asserted, 

 that the soft medulla of the nerve is lost in the muscular fibrils, 

 and incorporated with their substance (^ Monro de Nervis,^ § 22). 

 When a muscle is thrown into contraction by an impulse sent 

 along the nerve, its fibrils are contracted, and the two ends are 

 drawn together, so that the whole muscle is shortened; con- 

 sequently, it also draws the parts together to which, by means 

 of its tendons, it is attached. The latter are passive, and are 

 neither contractile, nor capable of receiving an impression. The 

 muscle may be moved either wholly or partly. The arteries 

 which are distributed to muscles are necessary to their com- 

 pleteness, so that without them they soon become unfit for 

 their functions, or diseased (129, iv) ; but the animal actions 

 themselves do not immediately cease when the influence of the 

 blood ceases {' Monro de Nervis,^ § 44), and are in no wise pro- 

 duced by it (Haller's 'Physiol.,' § 406). 



162. This action of the nerves on the muscles cannot be 

 explained by any of the laws of mechanical motion (Haller's 

 ' Physiol.,' § § 394, 412.) It is also manifest from all phenomena, 

 that the vibrations transmitted along the nerves, and com- 

 municated to the muscles, do not produce the movements of the 

 latter in any mechanical way (Ibid., §§ 376, 377); and as all 

 other methods of explaining mechanically these movements are 

 insufficient, we must turn to other moving forces. The move- 

 ment in the muscle which it receives from the nerves, only 

 takes place after an impression made on the nerve, — it may be 



