266 ANIMAL FORCES. [u. 



muscular movements may be either sentient or nerve-actions 

 only, or both at once. It is this common action of the two 

 kinds of vis nervosa in the muscular movements of animals 

 which renders the presence of the brain and its uninterrupted 

 connection with the muscles, necessary to the development of 

 many nerve-actions ; the co-operation of the cerebral forces or 

 of the conceptive force being unnecessary (494, ii). Haller 

 referring to the instinctive acts of animals, observes that the 

 causes of muscular action in these instances depend on a law 

 given by God, and not on the mind (^Physiology,' § 408). 



515. External sensations and other conceptions in many 

 ways change the action of the heart by means of their internal 

 impressions (167, 211) — that action being for the most part an 

 indirect nerve-action of external impressions on the heart (457, 

 459). Can internal impressions not caused by conceptions also 

 exercise a motor power over it ? It is a priori probable, for 

 the heart is a muscle. It is much more probable, however, 

 from another consideration deduced from experiment, namely, 

 that when in animals endowed with brain the cardiac nerves 

 are tied, the movements of the heart cease, even although all 

 the nerves are not tied at the same time (Haller's ' Physiology,' 

 § 100). Consequently, although the natural movements of the 

 heart be a direct nerve-action excited by external impressions, yet 

 it must be maintained by a co-operating internal impression, and 

 this must depend either on conceptions or other stimuli. Now, 

 although the ordinary natural stimuli of the heart's action be 

 unfelt, they are not the less sentient actions of other conceptions 

 (457 — 459). Perhaps the natural motion of the brain, which 

 synchronises with the respiratory movements, may be enume- 

 rated amongst these, although that motion is itself dependent 

 on the heart's action, and acts by communicating non-concep- 

 tional internal impressions to the cerebral origins of the cardiac 

 nerves ; yet, on the other hand, the heart beats in the foetus 

 in utero, or in the embryo in ovo, and also in animals which 

 have no respiratory movements. Or the heart's action may be 

 influenced by primary internal impressions, communicated to 

 the cardiac nerves by the arteries of the brain (505). In this 

 way we may understand why the connection of the brain with 

 the heart, is necessary to the movements of the latter, without 

 the cerebral forces being requisite. But reflected external im- 



