244 ANIMAL FORCES. [ii. 



movements. (See § 515.) It follows^ therefore^ that the heart's 

 action is, for the most part, purely a nerve-action of an external 

 impression, and direct, although, at the same time, a sentient 

 action also (167), especially of external sensations, and, therefore, 

 may be also an indirect nerve-action (421 — 423) . Consequently, 

 in animals without either sensation or brain, the movements of 

 the heart may go on just as in sentient animals, and be either 

 connatural or contranatural, according to the nature of the 

 external impressions exciting them; and the change thus in- 

 duced in the circulation may induce the same movements, 

 which, when the animal felt it, constituted incidental sentient 

 actions of the external sensation exciting the movements (453). 

 460. The pulse, or the contraction and dilatation of the 

 arteries, and the entire circulation of the blood through the 

 blood-vessels, are, for the most part, nerve-actions only, although 

 they may sometimes be also sentient actions (167, 205), and 

 indirect nerve-actions. They may take place independently of 

 the brain or of sensation. The action of the arteries, inde- 

 pendently of the heart, seems to be rather mechanical than 

 animal. They are endowed with the force of contractility, as 

 is shown when they contract on a finger inserted into them, 

 and when they contract again, so soon as they are distended 

 with the blood sent from the heart. This force is, in all pro- 

 bability, purely mechanical, for the arteries are always con- 

 tracted in the corpse ; and if fluids be injected, so as to distend 

 them, they contract again so soon as the pressure is withdrawn. 

 They have also a natural elasticity, for they retract very consi- 

 derably when divided. (Haller's 'Physiology,' § 33.) But they 

 have no sensibility, if their nerves be not irritated, which, how- 

 ever, are certainly not distributed to their fibrous tissue : even 

 Haller himself denies that they have any visible irritabihty. 

 {' Physiology,' § 32, and 'Opera Minora,' torn, i, pp. 377, 418.) 

 Nevertheless, they are surrounded with muscular fibres and 

 nerves, that have both properties. It appears, however, that 

 these are supplied by nature, to the end that their ordinary me- 

 chanical stroke may be changed by means of direct nerve-actions, 

 when certain external impressions, which are unusual or contra- 

 natural, are received. This is observed, when an artery, being 

 so wounded, that its nerves or muscular fibres are injured, be- 

 comes inflamed at the spot, and pulsates strongly. Probably 



