CH. II.] EXTERNAL DIRECT NERVE-ACTIONS. 243 



be also sentient actions, as has been already shown in detail, 

 in Part I. (Compare §§ 269, 285—293, &c.) 



455. The heart is a muscular viscus, which can be excited 

 into direct nerve-actions by an external impression (357), applied 

 either externally or internally; and even when it has been re- 

 moved from the body, and its action has already ceased for a 

 lengthened period. (Haller*s ' Physiology/ § 102.) 



456. Since the cardiac movements, thus excited, are neces- 

 sarily independent of the mind or the brain, and are, therefore, 

 in no respect sentient, they are direct nerve-actions excited by 

 an external impression, and would still be such if they were 

 also felt, and occurred (as they sometimes do) as sentient 

 actions of external sensations (452, 512). 



457. The external stimuli which more especially excite direct 

 nerve- actions in the heart, are the blood and other fluids, and 

 the air itself, if in contact with the inner surface. (Haller^s 

 ' Physiology,' § 101, and 'Opera Minora,' tom. ii, pp. 389, 390.) 

 Various stimuli, applied externally, have also the same effect ; 

 the heart is, indeed, the organ which, of all others, possesses 

 the greatest irritability (Haller's ' Physiology,' § 102) ; or, in 

 other words, which is the most readily excited to violent, direct 

 nerve- actions, by the greatest number of external impressions. 



458. Just as painful or pleasing sensations excite the heart's 

 action contrarily to, or in accordance with, its natural func- 

 tions (211, 204), so, also, are the nerve-actions, when excited by 

 those external impressions, which, if felt, would excite painful 

 or pleasing sensations (452). An excised heart is irritated to 

 convulsive movements, if the irritant be violent, or such as 

 would have been painful. Now, inasmuch as the circulation 

 of the blood is closely connected with the motions of the heart, 

 it may be either in accordance with the well-being of the 

 organism, or opposed to it, just as the external impressions on 

 the heart differ; and it thus appears that an abnormal com- 

 position or temperature of the blood renders the stroke of the 

 heart contranatural, in fevers and other diseases. 



459. The ordinary and natural stimuli, or the external im- 

 pressions of the heart, are not felt by animals (167); conse- 

 quently, their transmission beyond the heart is not necessary 

 to the excitation of its action, although it seems to be necessary, 

 in many animals, to the more certain maintenance of the 



