ANIMAL. 



133 



pear, slender as the means of accomplishing it 

 may seem to be, it is nevertheless essential : 

 interrupted for any length of time, the animal 

 inevitably perishes. A process of such im- 

 portance, as may be imagined, is not long left 

 without its appropriate and special apparatus. 

 This varies extremely in its structure, in the 

 different tribes of animals, and according to the 

 circumstances surrounded by which they live. 

 Some have lungs, branchiae or gills, and 

 tracheae opening by spiracula, of infinitely va- 

 ried construction. 



Respiration is also carried on vicariously 

 in a very large proportion of animals, if not 

 perhaps in all to a certain extent, by means 

 of the skin, and in some even by the instru- 

 mentality of the alimentary canal. 



The changes effected in the atmospheric 

 air by the respiratory apparatus of all animals 

 are similar, but they differ from those that are 

 produced by the corresponding implements in 

 vegetables : the proportion of oxygen it con- 

 tains universally diminishes, and the quantity 

 of carbonic acid gas it holds in solution as 

 invariably increases in amount. A quantity of 

 water or of watery vapour is at the same time 

 thrown off. This is exactly the opposite of 

 what we have seen to be the effect of respi- 

 ration among vegetables ; in these the quantity 

 of oxygen is augmented, whilst that of car- 

 bonic acid gas is diminished. The nutritive 

 fluids newly prepared by the apparatus of 

 digestion, or that have already gone the round 

 of the system, are by a variety of means ex- 

 posed, in the special or common apparatuses 

 mentioned, to the influence of the atmospheric 

 air, from the contact of which they undergo 

 certain important and often manifest changes 

 that fit them for their ultimate office in the 

 animal economy, the maintenance of its 

 parts, with their inherent capacities to execute 

 the various functions imposed upon them. 



The respiratory act among animals takes 

 place with the knowledge and with the assist- 

 ance and implied will of the individual. 

 Animals are informed of the necessity of re- 

 spiring by the feeling of a want, an uneasiness, 

 just as they are admonished of the necessity 

 of taking aliment by the painful sensations 

 denominated hunger and thirst. 



The essence of respiration in the two grand 

 classes of organized beings would therefore 

 appear to be different, and might be made 

 the ground of a definitive distinction between 

 the members of each kingdom. Carbon is the 

 object for which the respiration of plants is 

 instituted ; oxygen the end for which re- 

 lations are established between animals and the 

 atmosphere. Another grand difference be- 

 tween the respiration of plants and animals is 

 the involuntariness of the act in the one, and 

 its voluntariness in the other, its occurrence 

 with unconsciousness in the one, and with con- 

 sciousness in the other. 



The nutrient juices thus prepared have 



now to bo distributed ; this is done by means 



i peculiar motion impressed upon the fluids 



in virtue of a vital law with the nature of 



which we are still very imperfectly acquainted. 

 Let us use the word circulation in a sense 

 implying motion generally, not motion in a 

 circle to designate the act by which in the 

 organized world the nutritive juices are dis- 

 tributed through the frames of the objects 

 composing it. 



Circulation. There can be no doubt of the 

 existence of a circulation among vegetables ; in 

 many species currents in opposite directions 

 have even been seen with the aid of the micro- 

 scope, and this not only among the lowest and 

 most simple in their structure of the class, but 

 also in the highest and most complicated. The 

 circulation of vegetables appears to take place 

 within two different congeries of vessels, ex- 

 tremely numerous, and disposed according to 

 their nature in different parts of the plant. The 

 vessels that pump or transmit the sap from the 

 roots to the leaves, for instance, as we have 

 already had occasion to state, run within the 

 woody parts of plants ; those that receive the 

 modified juices of the leaves, again, take their 

 course downwards within the bark. These two 

 sets of vessels anastomose within the substance 

 of the leaves, but no where else; the second 

 set can alone be said to have a distribution 

 throughout the vegetable, for every part appears 

 to depend on them for its supply of nourish- 

 ment, even the extreme points of the roots, 

 which were themselves the first instruments in 

 collecting the aliment still unfit for the purposes 

 of nutrition. The best informed vegetable 

 physiologists are of opinion that the nutritive 

 fluid once sent off from the leaves never finds 

 its way back to these organs again ; it is ab- 

 sorbed or fixed by the different parts or struc- 

 tures to which it is distributed, ministering to 

 their increment generally, and enabling each to 

 manifest its specific function in the vegetable 

 economy. 



In this motion of the fluids of vegetables it 

 is evident that there is little analogous to what 

 we find within the bodies of animals somewhat 

 elevated in the scale. But let us first cast 

 a hasty glance at what does take place within 

 this other division of the organic kingdom be- 

 fore instituting a comparison between the func- 

 tions of circulation in the two. All animals, 

 from the mammalia downwards to the entozoa, 

 birds, reptiles, fishes, the mollusca, Crustacea, 

 arachnida, insecta, and, among the radiata, the 

 holothuriae, echini, and asteriae, include within 

 their organisms particular canals or vessels for 

 containing and distributing their nutrient juices, 

 and within which, moreover, these are in motion 

 in a circle. In the acalephae we still find canals 

 branching off from the digestive cavity and dis- 

 tributing the nourishment there prepared to the 

 different parts of the body : in these, however, 

 we no longer find any contrivance for establish- 

 ing a circular motion in the nourishing juices. 

 Still lower in the scale, among the polypes and 

 actineae, for example, we discover no branched 

 appendages or canals for the distribution of the 

 nutrient fluids ; those prepared in the stomachs 

 of the animals appear to penetrate their sub- 

 stance directly, and to permeate the homo- 

 geneous cellular tissue of which they consist. 



