88 FUNCTIONS OF THE NERVOUS SYSTEM. 



usually most odoriferous*), smell may be regarded, as taste also is probably 

 to be considered, in the light of a refined kind of touch. 



104. 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 existence of 

 gases ; and we notice the prevalence of the same general fact in the Animal 

 as in the Organic life. We cannot become cognizant of the changes, or even 

 of the existence, of the external world, unless some material effect be produced 

 by it on our organs of sense ; nor can we produce any alteration in its condi- 

 tion, except by powers which act according to purely mechanical principles. 



105. In regard to the Muscular System, it has already been sufficiently ex- 

 plained 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 

 its due supply of arterial blood ; and that the complete separation of any mus- 

 cular part from all its nervdpB connections, has none but an indirect influence 

 on its properties. 



CHAPTER III. 



FUNCTIONS OF THE NERVOUS SYSTEM. 



I. General Summary. 



106. ALL our positive knowledge of the functions of the Nervous System, 

 is derived from observation of the movements exhibited by animals, and from 

 our own consciousness of what passes within ourselves. Except through the 

 movements consequent upon them, we have no means of ascertaining whether 

 or not particular changes in the nervous system are attended with sensation. 

 The cries and struggles of the animal made the subject of experiment, are 

 ordinarily considered as indications of sensations ; but it is not right so to re- 

 gard them in every instance; nor are we justified in asserting that conscious- 

 ness results from any external irritation, merely because movements evidently 

 tending to get rid of this are performed in respondence to it. We know that 

 the contractions of the heart and alimentary tube are ordinarily excited by a 

 stimulus, without any sensation being involved ; and these movements, like 



* Some of the most strongly odoriferous substances, however, are solid; for instance, 

 musk: and it has been experimentally proved, that the loss of weight, which follows the 

 free exposure of a minute quantity of this perfume to an atmosphere constantly renewed 

 during several years, is not appreciable by the finest balance. 



