562 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



purely physical, and is termed respiration. Respiration may therefore 

 be defined as that function by means of which oxygen is taken into the 

 system and carbon dioxide eliminated. Oxygen passes into the blood 

 from the air, mainly by a process of gaseous diffusion ; the same process 

 is also largely concerned in the giving up of oxygen by the blood to the 

 different tissues and the absorption of carbon dioxide from the different 

 tissues by the blood, and the yielding of this gas to the air within the 

 lungs. 



I. General View of the Organs of Respiration. — The function of 

 respiration is an organic function, existing in the vegetable as well as in 

 the animal, the products of respiration being the same in both. In the 

 vegetable the introduction of oxygen and elimination of carbon dioxide 

 takes place both during day and night, but during the day the function 

 of respiration is masked by the processes of nutrition in which the 

 reverse change takes place, carbon dioxide being appropriated by the 

 plant and oxygen set free. 



In examining the different forms of respiratory apparatus peculiar 

 to different kinds of organic beings it is readily determined that in all 

 the main respiratory tissue is that part of the respiratory apparatus 

 which is concerned in the absorption of oxygen and in the elimination 

 of carbon dioxide, and, like the digestive tube, is simply a modification 

 of the external tegnmentary surface. 



In the simplest forms of plant life the external tegnmentary covering 

 constitutes the entire respirator apparatus, and it is through this that 

 oxygen is absorbed and carbon dioxide eliminated. But in the more 

 highly organized forms of plants a system of tubes, known as the spiral 

 vessels, are found ramifying through their stems and leaves, and even in 

 their most perfect form seldom contain other than gaseous matters. 

 These spiral vessels in the endogenous plants are universally distributed 

 through the stem, and form a part of every bundle of fibro- vascular 

 tissue; but in the exogenous plants they are usually confined to the 

 medullary sheath immediately surrounding the pith. In each case, how- 

 ever, they traverse the stems in such a manner as to enter the leaves 

 through the foot-stalks. These vessels carry air from the exterior to the 

 interior, and it is an especial fact to be noted that the atmospheric air 

 contained in them possesses a larger amount of oxygen than the free 

 atmosphere. On the under surfaces of leaves are also to be found 

 distinct openings, or stomata, which, although apparently for the admis- 

 sion of materials necessary for nutrition, also seem to have the power 

 of admitting air into the cavities existing in the leaves, especially beneath 

 the inferior cuticle. Thus, it is the external surface of the plant through 

 which respiration is practically performed. 



So, also, in animals, respiration always takes place through an 



