RESPIRATION. 



life, while it is inseparably connected with its fellow-orbs, performing a special and ye 

 a collective work in the vast body to which it belongs ; darkening and brightening in 

 its alternate night and day until it has completed it career. 



In order to prevent other organs from pressing on the heart, and so preventing it 

 from playing freely, a membranous envelope, called from its office the " pericardium," 

 surrounds the heart and guards it. 



The various operations which are simultaneously conducted in our animal frame are 

 so closely connected with each other that it is impossible to describe one of them 

 without trenching upon the others. Thus, the system of the circulatory movement by 

 which the blood passes through the body, is intimately connected with the system o: 

 RESPIRATION, by which the blood is restored to the vigor needful for its many duties. 

 In order to renew the worn-out blood, there must be some mode of carrying off its 

 effete particles, and of supplying the waste with fresh nourishment. For this purpose 

 the air must be brought into connection with the blood without permitting its escape 

 from the vessels in which it is confined. The mode by which this object is attained, in 

 the Mammalia, is briefly as follows : 



A large tube, appropriately and popularly called the " windpipe," leads from the back 

 of the mouth and nostrils into the interior of the breast. Just as it enters the chest 

 it divides into two large branches, each of which subdivides into innumerable smaller 

 branchlets, thus forming two large masses, or lobes. In these lobes, or lungs, as they 

 are called, the airbearing tubes become exceedingly small, until at last they are but 

 capillaries which convey air instead of blood, each tube terminating in a minute cell. 

 The diameter of these cells is very small, the average being about the hundred and 

 fiftieth of an inch. Among these air-bearing capillaries the blood-bearing capillaries 

 are so intermingled that the air and blood are separated from each other only by 



membranes so delicate that the 

 comparatively coarse substance of 

 the blood cannot pass through, al- 

 though the more ethereal gases 

 can do so. So, by the presence 

 of the air, the blood is renewed 

 in vigor, and returns to its bright 

 florid red, which had been lost in 

 its course through the body, while 

 the useless parts are rejected, and 

 gathered into the air-tubes, from 

 whence they are expelled by the 

 breath. 



The accompanying illustra- 

 tions will give a good idea of the 

 capillary structure. Fig. i rep- 

 resents the air-tubes of the lungs, 

 and fig. 2 exhibits the capillaries 

 through which the blood is con- 

 veyed. 



The heart is placed between the two lobes of the lungs, and is in a manner embraced by 

 them. The lungs themselves are enclosed in a delicate membrane called the "pleura." 

 These two great vital organs are situated in the breast, and separated from the diges- 

 tive and other systems by a partition, which is scientifically known by the name of 

 " diaphragm," and in popular language by the term " midriff." This structure does 

 not exist in the Birds ; and its presence, together with that of the freely-suspended 

 lungs, is an unfailing characteristic of the Mammalian animal. 



Thus the entire structure bears the closest resemblance to a tree, growing with its root 

 upwards and its leaves downward, the trachea being the trunk, the branchial tubes 

 the limbs, the smaller tubes are the branches, and the air-cells the leaves. A similar 

 idea runs through the nerve system and that of the blood; all three being interwoven 

 with each other in a manner most marvellous and beautiful. 



FIG. i. 



AIR-TUBES OF THE LUNGS. 



FIG. 2. 



CAPILLARIES OF THE LUNGS. 



