CIRCULATION OF NUTRITIVE FLUID. 305 



organs through which they have heen transmitted, have undergone very 

 different changes by the nutritive and secreting processes, according to 

 the function of the organs which they have supplied ; and if the same 

 portion of the circulating fluid were constantly being transmitted to 

 each organ, and returned from it, its composition would speedily undergo 

 a change that would render it no longer fit for its purposes. By the 

 union of the different local circulations, however, into one general circu- 

 lation, this change is prevented, and the whole mass of the blood is 

 maintained in its normal or regular condition; for as its composition is 

 such, as to supply all parts of the body, in a state of health, with the 

 proportions of nutritive material which they respectively need ; and as 

 the returning currents are all mingled together in the vessels, before 

 being again distributed to the system, each part supplies what the other 

 has been deprived of, and thus the normal proportion of ingredients in 

 the whole mass of the blood is constantly kept up, whilst in each of its 

 separate streams it is undergoing an alteration of a different kind. 



539. But these processes alone might be carried on by the aid of a 

 much less rapid Circulation, than that which exists in Man and the 

 higher animals. We do, in fact, occasionally meet with examples in 

 which they continue for some time, under an almost total stagnation of 

 the current. There are others, however, which require a much more 

 rapid and uninterrupted movement of the circulating fluid. We have 

 already seen that, for the action of the Nervous and Muscular tissues, 

 oxygen is necessary ; and the amount of that gas contained in the blood 

 circulating through these tissues would be very speedily exhausted, if it 

 were not continually renewed ; whilst the carbonic acid, which is formed 

 at the expense of that oxygen, would speedily accumulate to an injurious 

 degree, if it were not carried off as fast as it is produced. Hence we 

 find that, in all Animals, the maintenance of the Respiration, by carry- 

 ing Oxygen from the respiratory surface into the different parts of the 

 system, and by conveying back Carbonic acid to be thrown off at the 

 Respiratory surface, is one of the great purposes of the Circulation of 

 the blood ; and its extreme importance is shown by the very speedy 

 check, which the interruption of this function produces in the movement 

 of the blood, in warm-blooded animals. Thus in a Bird or Mammal, 

 completely cut off from Oxygen, the circulation in the lungs will come 

 to a stop, which stoppage will necessarily extend itself over the whole 

 body, in little more than three minutes. We find, then, that the rate 

 of the Circulation in different animals bears a relation to the energy of 

 their Respiration ; and this energy is closely connected with the general 

 activity of their functions, but particularly with that of the Nervous and 

 Muscular systems, which are most dependent for the exercise of their 

 powers upon a continually fresh supply of oxygen, and upon the un- 

 ceasing removal of the carbonic acid which is generated in their sub- 

 stance. 



2. Different forms of the Circulating Apparatus. 



540. It is desirable that the Circulating apparatus should be first 

 studied in its very simplest form, that which it possesses in Plants and 



20 



