268 ANIMAL FORCES. [ii. 



of the cardiac nerves are peculiar, and the action of impressions on 

 the heart differs from that on other muscles. The number of the 

 cardiac nerves, their origin from such different points, their in- 

 tricate combination, and their ganglia and remarkable plexus 

 all merit the closest attention. Probably, the results of external 

 impressions would be very different, according as the nerves 

 were irritated, or tied, or divided, above or below the ganglia 

 and plexuses and points of division into branches ; and that the 

 different results observed when the cardiac nerves are imtated 

 — the hearths action being sometimes increased, sometimes un- 

 altered — may be dependent upon some such difference in the 

 mode of experimenting (Haller^s 'Physiology,^ loco citato). 



517. Since the heart is compounded of several muscles, and 

 since these can be excited to action independently of each 

 other, as has been shown in excised hearts, it is more probable, 

 that the stroke of the heart, or the combined action of all the 

 muscular structures, is an indirect nerve-action of external im- 

 pressions, they being reflected in the heart itself (513), than 

 that it is simply a mechanical result of their direct nerve- 

 actions (447); for in the latter case, the whole heart must be 

 excited to action by an external impression which acts only on 

 certain of its fibres — a conclusion opposed to the results of ex- 

 periments (Haller's 'Physiology,' § 101). 



518. Although the preceding propositions are well founded, 

 still many a change in the heart's action is either a sentient ac- 

 tion, or at the same time both a sentient and a nerve-action; and 

 consequently, it is erroneous to infer, because the heart's action 

 is usually a direct nerve-action of external impressions, or in 

 other words, a result of irritability, that it is always such,' or 

 such only, or such in all animals. Lastly, in so far as the 

 circulation is dependent on the heart, each kind of impression 

 contributes something thereto, so that the whole of the circula- 

 tion derives some stimulus to its continuance and maintenance 

 from every kind of animal motor force, although the customary 

 and principal stimuli consist in unfelt external impressions (459). 



519. The arterial pulse, in so far as it is dependent on the 

 heart's action and the circulation, may be a nerve-action of 

 every kind (518). Many of the stimuli used in cases of 

 suspended animation to restore life by re-exciting the action of 

 the heart, the circulation, and the pulse, act as non-concep- 



