PHYSIOLOGY. 



sensation ; since volition, cannot be con- 

 ceived without desire, and without a feel- 

 ing of pleasure or pain. The goodness, 

 which we observe in all the works of na- 

 ture, will not allow us to believe that she 

 has formed beings with the power of sen- 

 sation, that is, with a susceptibility of 

 pleasure and pain, without enabling them 

 at the same time to approach to the one 

 and fly from the other, at least to a cer- 

 tain degree. And if, among the too real 

 misfortunes which afflict our species, one 

 of the most affecting is the sight of a 

 man of sensibility deprived by superior 

 force of the power of resisting oppression ; 

 the poetic fictions, most apt to excite our 

 pity, are those which represent sensible 

 beings inclosed in immovable bodies ; 

 and the tears of Clorinda, flowing with 

 her blood from the trunk of a cypress, 

 ought to arrest the blows of the most 

 savage man. 



Independently of the chain, which 

 unites these two powers, and of the 

 double apparatus of organs which they 

 require, they produce also several modi- 

 fications in the faculties common to all 

 organized bodies ; and these modifica- 

 tions, joined to the two peculiar powers, 

 constitute more particularly the essential 

 nature of animals. Thus, in respect to 

 nutrition, vegetables being attached to 

 the earth, absorb nutritive fluids directly 

 by their roots ; these almost infinitely sub- 

 divided, penetrate the smallest intervals 

 of the soil, and, if we may use the ex- 

 pression, travel to a distance in quest of 

 nourishment for the plant to which they 

 belong ; their action is quiet and constant, 

 being liable to interruption only when 

 drought deprives them of the necessary 

 juices. Animals, on the contrary, fixed 

 to no spot, but frequently changing their 

 abode, required the power of transporting 

 with them the provision of fluids neces- 

 sary for their nutrition; they have there- 

 fore an interior cavity to receive their 

 food ; and on its inner surface there are 

 the openings of absorbing vessels, which, 

 to use the energetic language of Boer- 

 have, are real internal roots. The size of 

 this cavity, and of its orifices, allowed in 

 several animals the introduction of solid 

 substances. These required instruments 

 for their division, and liquors for their so- 

 lution ; in a word, nutrition was no longer 

 performed by 'the immediate absorption 

 of matters in" the state in which the earth 

 or atmosphere furnished them ; it was 

 necessarily preceded by various prepara- 

 tory operations, which, taken altogether, 

 constitute digestion. 



Thus digestion is a function of a secon* 

 dary class, peculiar to animals. Its ex- 

 istence, as well as that of the alimentary 

 cavity in which it takes place, is render- 

 ed necessary by the power which animals 

 have of voluntary motion ; but it is not 

 the only consequence of that power. 



Vegetables, having few faculties, are 

 simple in their organization ; being com- 

 posed almost entirely of parallel or slightly 

 diverging fibres. Moreover, their fixed 

 position admitted of the general motion of 

 their nutritive fluid being kept up by sim- 

 ple external agents ; thus it ascends by 

 means of suction in their spongy or ca- 

 pillary texture, and also through the in. 

 fluence of evaporation, from the surface ; 

 it is rapid in a direct ratio to this evapo- 

 ration, and may even become retrograde 

 when that process ceases, or when it is 

 changed into absorption by the moisture 

 of the atmosphere. 



It was necessary that animals should 

 have within themselves an active princi- 

 ple of motion for their nutritive fluid, not 

 only because they were destined to con- 

 stant changes of situation and tempera- 

 ture ; but also from their more numerous 

 and highly developed faculties requiring 

 a much greater complication of organs. 

 Hence the component parts became very 

 intricate in their composition, and often 

 very distant, and possessed in many in- 

 stances a power of changing their relative 

 position, consequently the means of carry- 

 ing the nutritive fluid through s'iich mul- 

 tiplied intricacies must be more powerful 

 than in vegetables, and differently arrang- 

 ed. It is contained, in most animals, in 

 innumerable canals, which branch out 

 from two trunks, that communicate to- 

 gether in such a way, that the fluid urged 

 into the branches of one is received by 

 the roots of the other, and carried back 

 to a common centre, from which it is pro- 

 pelled afresh. 



At the point of communication between 

 the two great trunks is placed the heart, 

 whose contractions impel the nutritive 

 fluid into all the branches of the arterial 

 trunk ; for the orifices of the heart pos- 

 sess valves, disposed in such a way that 

 the circulating juices can only move in 

 the directions now described, viz. from 

 the heart towards all parts by the arte- 

 ries, and from all parts to the heart in 

 the veins. 



In this rotatory motion consists the cir- 

 culation of the blood, which is another 

 secondary function peculiar to animals, 

 chiefly performed and regulated by the 



