THE CIRCULATION. 109 



How far the globulin of the red corpuscles is concerned in 

 nutrition, is quite unknown. It has been inferred that the 

 phosphuretted fat contained in them may be concerned 

 particularly in the nutrition of nervous matter, which 

 contains a somewhat similar compound; and that the 

 potash, which is in larger proportion in the red corpuscles 

 than soda, indicates an especial relation to the nutrition 

 of muscle, in which also potash preponderates. 



The relation of the red to the white corpuscles of the 

 blood has been already considered; of the functions 

 of the latter, other than are concerned in this relation- 

 ship, nothing whatever is known. 



CHAPTEE VI. (Continued). 



CIECULATIOlSr OF THE BLOOD. 



THE body is divided into two chief cavities, the chest or 

 thorax and abdomen, by a curved muscular partition, called 

 the diaphragm (fig. 31). The chest is almost entirely filled by 

 the lungs and heart ; the latter being fitted in, so to speak, 

 between the two lungs, nearer the front than the back of the 

 chest, and partly overlapped by them (fig. 31). Each of 

 these organs is contained in a distinct bag, called respec- 

 tively the right and left pleura and the pericardium, which 

 is fibrous in the main, but lined on the inner aspect by a 

 smooth shining epithelial covering, on which can glide, 

 with but little friction, the equally smooth surface of the 

 organ enveloped by it. In fig. 3 1 the containing bags of 

 pleura and pericardium are supposed to have been removed. 

 Entering the chest from above is a large and long air- tube, 

 called the trachea, which divides into two branches, one 

 for each lung, and through which air passes and repasses 

 in respiration. Springing from the upper part or base of 

 the heart may be seen the large vessels, arteries, and 

 veins, which convey blood either to or from this organ. 



