FUNCTIONS OF ANIMAL LIFE. 235 



little impaired, except through the incapability, on the part of any piece of 

 human mechanism, to imitate those wondrous contrivances of Infinite Skill, 

 which have for their object the adaptation of the instrument to varieties of dis- 

 tance, of intensity of light, &c. There can be little doubt, that the structure 

 of the Ear is arranged to do the same for the sonorous vibrations, which the 

 eye does for the rays of light; that is, through its means, the undulations 

 which strike upon the external surface of the organ are separated and distin- 

 gished, those of a like kind being brought together upon one division of the 

 nerve, and those of another order upon a different set of fibres ; so that the 

 different kinds of sound, and the peculiar quality and direction of each, may 

 be discriminated ; whilst, by the concentration of all the impressions of the 

 same character, a higher amount of force is given to them. Of the sense of 

 Smell, no similar account can be given ; since the medium by which odours are 

 propagated is not known. If, as is generally believed, this is accomplished by 

 the diffusion through space, of minute particles of the odoriferous body itself 

 (which supposition seems to derive support from the general fact, that the 

 most volatile substances are usually most odoriferous), smell may be regarded, 

 as taste also is probably to be considered, in the light of a refined kind of 

 touch. 



287. Thus, the general rule holds good, here as elsewhere, that the pro- 

 cesses, by which the organism is immediately brought into relation with the 

 external world, are performed in obedience to physical laws ; the living struc- 

 ture only affording certain peculiar conditions, which may be imitated in a 

 great degree by other means. This is the case, for example, with regard to 

 Digestion, which is in itself a simply Chemical process, that will take place 

 out of the body as well as in it, if the materials and the necessary solvent be 

 submitted to the same circumstances, as those to which they are exposed in 

 the stomach ; and in regard also to the act of Respiration, which depends 

 upon the physical tendency to mutual diffusion, inseparable from the exist- 

 ence of gases : and we notice the prevalence of the same general fact in the 

 Animal as in the Organic functions. We cannot become cognizant of the 

 changes, or even of the existence, of the external world, unless some mate- 

 rial effect be produced by it on our organs of sense ; nor can we produce any 

 alteration in its condition, except by powers which act according to purely 

 mechanical principles. 



288. In regard to the Muscular System, it has already been sufficiently 

 explained that it forms a part of the apparatus of Animal life, no otherwise 

 than as the instrument by which nervous energy operates upon external 

 objects. The contractility which it manifests on the application of a stimulus, 

 is an endowment which it derives from its own structure, and not from the 

 nervous system ; for it will be clearly proved in its appropriate place, that 

 the presence of this contractility is connected with the healthy nutrition of 

 the tissue, and with its due supply of arterial blood ; and that the complete 

 separation of any muscular part from all its nervous connections, has none 

 but an indirect influence on its properties. 



