ON RESPIRATION IN GENERAL. 389 



retention would exert on the welfare of the system at large. Of all these 

 injurious ingredients, Carbonic Acid is without doubt the most abundantly 

 introduced into the nutritive fluid ; and it is also most deleterious in its etiects 

 on the system, if allowed to accumulate. One of the most important changes 

 which result from its retention, is the stagnation of the blood, both in the sys- 

 temic and pulmonary capillaries ; for there is evidence that, if the process of 

 aeration, by which the venous blood brought to the lungs is converted into 

 arterial, be in any way checked, the flow of blood through the pulmonary 

 capillaries speedily ceases ; and that if venous blood be propelled through the 

 system, in place of arterial, it is transmitted with difficulty through the sys- 

 temic vessels. The cause is the same in both instances; the normal changes, 

 which the blood ought to undergo in these vessels, are prevented ; and there 

 is consequently a cessation of that capillary power which has been shown to 

 be one of the most important of the forces by which the blood is kept in 

 motion ( 511). 



521. We find, accordingly, that the provision for the removal of carbon 

 from the blood, surpasses in extent that which is made for any other excretion. 

 The two largest glands in the body the Liver and the Lungs are designed 

 for this purpose ; but their operation is made subservient, in each case, to other 

 objects. By the Liver, the carbon is excreted, with other elements, in the 

 form of a fluid, which has important uses in the digestive function ; whilst by 

 the Lungs (which will be presently seen to have in all essential points a glan- 

 dular structure) it is thrown off in a gaseous form, and thus is made subser- 

 vient, according to the laws of the mutual diffusion of gases, to the introduction 

 of oxygen into the system, and consequently to the maintenance of the animal 

 temperature, as well as of the stimulating properties of the blood. It is evi- 

 dent, then, that any circumstances which check the excretion of carbonic acid 

 by the lungs, will have an immediately injurious effect upon the system at 

 large; by causing the accumulation, in the fluid upon which it is. dependent 

 for the performance of its vital actions, of an agent that so seriously injures its 

 vivifying properties. But this is not the only mode in which the cessation 

 of this function becomes injurious. The exclusion of a constant supply of 

 oxygen from the blood, even though the removal of the carbonic acid were 

 provided for by other means, deprives it of its due power of nourishing and 

 exciting to action the tissues and organs to which it is afterwards distributed ; 

 for it would appear that this element is, throughout animated nature, a stimu- 

 lant as essential to the energy of its operations, as caloric is to all, and light to 

 many of these. Further, in those animals in which (as in Man) the whole 

 current of blood passes through the Respiratory apparatus, any stagnation in 

 its capillaries must derange, and soon check, the systemic circulation. There 

 are some animals, however, (such as Reptiles,) in which only a portion of the 

 blood that has returned from the system is transmitted to the lungs by each 

 impulse of the heart; so that their pulmonary circulation is in some respects 

 upon the footing of the portal circulation in other animals: in such, therefore, 

 the interruption of the pulmonary circulation will not immediately suspend 

 the movement of the blood through the systemic vessels; and in the Batrachia, 

 whose soft moist skin allows the air to act with tolerable freedom upon the 

 blood contained in its vessels, life may be prolonged for a considerable time, 

 even after the complete removal of the lungs, provided the temperature be 

 low.* But if, under these circumstances, the skin be covered with any unc- 

 tuous substance, preventing the transmission of air, death speedily ensues. 



522. The necessity for the Aeration of the Circulating fluid, is most remark- 



* See Edwards on the Influence of Physical Agents on Life (Translation by Hudgkin), 

 p. 32. 



33* 



