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MUTUAL DEPENDENCE OF VITAL OPERATIONS. 223 



265. The regular maintenance of the functions of Animal life is thus entirely 

 dependent upon the due performance of the Nutritive operations; a considera- 

 tion of great importance in practice, since a very large proportion of what are 

 termed functional disorders (of the Nervous system especially) are immediately 

 dependent upon some abnormal condition of the Blood. But there also exists 

 a connection of an entirely reverse kind, between the Organic and Animal func- 

 tions; for the conditions of Animal existence render the former in great degree 

 dependent on the latter. Thus, in regard to the acquisition of food, the Animal 

 has to make use of its senses, its psychical faculties, and its power of locomotion, 

 to obtain that which the Plant, from the different provision made for its support, 

 can derive without any such assistance. Moreover, the propulsion of the food 

 along the alimentary canal is effected by a series of operations, in which the 

 Nervous and Muscular systems are together involved at the two extremes; 

 though simple Muscular contractility is alone employed through the greater 

 part of the intestinal canal. Thus, the change in the conditions required for 

 the ingestion of food by Animals, has rendered necessary the introduction of 

 an additional element in the apparatus, to which nothing comparable was to 

 be found in Plants. Again, in the function of Respiration as performed in the 

 higher Animals, the Nervous and Muscular systems are alike involved; for the 

 movements, by which the air in the lungs is being continually renewed, are 

 dependent upon the action of both ; and those by which the blood is propelled 

 through the respiratory organs, are chiefly occasioned by the contractility of a 

 muscular organ, the heart. But in regard to the simple contractility of mus- 

 cular fibre, upon the direct application of a stimulus to it, which is the agent 

 in the movements of the heart and of the alimentary canal, it may be remarked, 

 that it does not differ in any essential particular from that which is witnessed 

 in many Vegetables : so that it strictly belongs to the functions of Organic life. 

 And with respect to those concerned in the act of Respiration, as well as those 

 which govern the two orifices of the alimentary tube, it will hereafter appear 

 that they result, equally with the former, from the application of a stimulus ; 

 and that they may be performed without any consciousness on the part of the 

 individual (though ordinarily accompanied by it) : the difference being, that 

 in the former the stimulus is applied to the contractile part itself, whilst in the 

 latter it is applied to an organ with which this is connected by nerves only. 

 Now we have, even in Vegetables, instances of the propagation of an irritation 

 from one part to another, so that a motion results in a part distant from that 

 stimulated, as in the case of the Sensitive Plant or Venus's Fly-trap. The 

 only essential difference, therefore, between those movements of Animals, 

 which are thus closely connected with the maintenance of the organic func- 

 tions, and those of Plants, consists in the medium through which they are 

 performed, this being in Animals a distinct Nervous and Muscular apparatus, 

 whilst in Plants it is only a peculiar modification of the ordinary structure. 



266. From what has been said, then, it appears that all the functions of the 

 Animal body are so completely bound up together, that none can be suspended 

 without the cessation of the rest. The properties of all the tissues and organs 

 are dependent upon their regular Nutrition, by a due supply of perfectly-ela- 

 borated blood ; this cannot be effected unless the functions of Circulation, 

 Respiration, and Secretion, be performed with regularity, the first being ne- 

 cessary to convey the supply of nutritious fluid, and the two latter to separate 

 it from its impurities. The Respiration cannot be maintained without the in- 

 tegrity of a certain part of the nervous system; and the due action of this, 

 again, is dependent upon its regular nutrition. The materials necessary for 

 the replacement of those which are continually being separated from the blood, 

 can only be derived by the Absorption of ingested aliment ; and this cannot 

 be accomplished without the preliminary process of Digestion. The intro- 



