242 ANIMAL FORCES. [ii. 



conceptions (163), still an infinite number of instances are ob- 

 served in the natural state, in which they take place solely 

 by means of the vis nervosa, and, in particular, that of external 

 impressions. A limb is often moved, not only against our 

 wish, but often without any feeling of the irritation that causes 

 the involuntary movement. It often happens that an impres- 

 sion in the intestinal canal, which is not felt, causes, in virtue 

 of an indirect nerve-action, the most violent movements of the 

 limbs; as is proved by epileptic paroxysms dependent on worms, 

 and the presence of which, in the intestinal canal, is not indi- 

 cated to either the physician or patient, by any peculiar phe- 

 nomena (470). Thus, also, an irritant poison causes spasmodic 

 action of the intestines, which is only indicated by the rumbling 

 of flatus, but not felt. So, also, the stimulus of light causes 

 contraction of the iris, as an indirect nerve-action, without any 

 accompanying sensation. Consequently, there are many move- 

 ments considered to be sentient actions of external sensations 

 only, which are nevertheless, at the same time, direct or indirect 

 nerve-actions of an external impression which is felt; and in 

 this way may be explained many movements made during sleep, 

 particularly by somnambulists, and those habitual voluntary 

 movements which are induced by external impressions that are 

 not felt. But the principal point is, that on this depends the 

 secret of the instincts in those animals which probably do not 

 feel the sensational stimuli of the instincts ; as, for example, 

 animals in utero, or in ovo, which already know how to aid 

 their birth before they seem to have felt anything; or animals 

 which are stimulated to undertake movements the most skil- 

 fully adapted to their preservation, without having been taught 

 by experience (269) ; or animals which, during their whole life, 

 do nothing that shows the least trace of ideas, and, conse- 

 quently, of external sensations, as polypes and oysters, &c. 

 In all these examples, the external impressions which are ordi- 

 narily the sensational elements of the instincts, seem to act as 

 a vis nervosa in producing the movements of the so-called in- 

 stincts; and many of these are undoubtedly true, direct, nerve- 

 actions, although it is equally certain that others are indirect. 

 In animals which gradually learn to feel, and to form volitions, 

 these external impressions become, in time, to be external sen- 

 sations, and the movements, hitherto nerve-actions, become to 



