cH. III.] VIS NERVOSA OF INTERNAL IMPRESSIONS. 271 



of the heart's movement, the co-operation of an internal im- 

 pression is necessary to its maintenance, and probably with the 

 same object (515), for so soon as the phrenic nerve is tied, the 

 movement of the diaphragm is interrupted (171). 



525. Although the motion of the diaphragm is influenced 

 by volitional conceptions and external sensations, still its 

 natural action in respiration is a nerve-action. Nevertheless, 

 while the cerebral forces and the mind are not requisite to that 

 action, the connection of its nerves with the brain is, inasmuch 

 as non-conceptional internal impressions co-operate in exciting 

 it. This doctrine is equally applicable to the respiratory move- 

 ments, in which the diaphragm plays so important a part. The 

 morbid changes in the respiratory movements arising from 

 contra-natural nerve-actions in distant parts, especially the ab- 

 domen, sufficiently prove this influence of the vis nervosa of 

 non-conceptional internal impressions on the nerves of the 

 thoracic muscles, and of the structures subservient to respira- 

 tion. 



526. We can hence confirm the proposition already mooted 

 (285) as to the respiratory function ; namely, that at first in the 

 newly born, it is a nerve-action of external impressions, or at 

 the most a sentient action excited by obscure external sensa- 

 tions (525) ; that subsequently it continues both as a sentient 

 action excited by an instinct originating in those obscure ex- 

 ternal sensations (364, 285), but being also, from the habitual 

 recurrence of those stimuli, a direct or indirect nerve-action of 

 external impressions (51), occurring mechanically (475) ; and is 

 continually changed into a sentient action by new instincts, or 

 volitional conceptions, constituting the acts of laughing, weep- 

 ing, sighing, singing, speaking, &c. (285). According to all 

 probability, this is the true nature of the respiratory move- 

 ments in animals endowed with consciousness. In those not 

 so endowed, their mechanism is altogether diff'erent, and they 

 consist solely of nerve-actions. 



527. The skin and mucous membranes have not a structure 

 capable of movements from external sensations, and, conse- 

 quently, cannot manifest nerve-actions from non-conceptional 

 internal impressions. Their vessels, and the glandular struc- 

 tures imbedded in them are, however, capable of their proper 

 nerve-actions. (Compare §^ 520 — 522.) 



