CH. III.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 267 



pressions may constitute this co-operating force in the action of 

 the heart ; the external impressions being reflected downwards 

 in the ganglia on their way to the brain^ or changed into internal 

 impressions in the spinal cord, and so acting reflexly in the 

 heart ; for it is quite certain that external impressions made on 

 the cardiac nerves are transmitted upwards to the brain, be- 

 cause they are sometimes felt (35). The object of nature in 

 thus providing that internal impressions shall co-operate in the 

 natural movement of the heart, although external impressions 

 alone may fully efi'ect it (457), seems to be simply this, that 

 the continuance of an action so important to life may be the 

 more certainly maintained, and in the less danger of being 

 interrupted than it would otherwise be, if wholly dependent 

 upon the usual external impression, namely, the stimulus of the 

 inflowing blood on the heart. When this ceases momentarily, 

 or acts only feebly, as in syncope, still the heart is not left 

 without a stimulus to its continued action. Observation seems 

 to corroborate this opinion, for the enfeebled or interrupted 

 stroke may be restored or invigorated in a moment by the ex- 

 ternal impression of irritating vapours on the nerves of the 

 nose, and which are reflected upon the cerebral origin of the 

 cardiac nerves. It is not probable, that any other end is 

 gained from this co-operation of internal impressions, for they 

 only strengthen the motion of the heart, and if it be interrupted, 

 a slight irritant re-excites it, as has been shown by experiments, 

 when by means of an irritation (a non-conceptional internal 

 impression) applied to the eighth pair, to the brain, or to the 

 spinal cord, the stroke of the heart has been rendered stronger 

 (Haller's ' Physiology,^ § 100), although by no means able to 

 maintain it continuously. Farther, although the ligature or 

 section of the cardiac nerves enfeebles the heart's action, it 

 never quite abolishes it (Haller, loco citato) ; whilst a slight ex- 

 ternal impression (its principal excitant) will re-excite the 

 movement. 



516. Without, however, entering further into this obscure 

 part of the subject, we conclude that the heart is susceptible 

 of nerve-actions from non-conceptional internal impressions, 

 although the principal motive force consists in unfelt external 

 impressions (459). Additional proofs will be found in § 519. 

 Further researches are much to be desired, for the properties 



