CIRCULATION, RESPIRATION AND EXCRETION 139 



we find that substantially the same stimuli which cause more 

 active breathing also stimulate the heart action and vice versa. 

 Lack of oxygen or excess of carbon dioxid are the two prin- 

 cipal factors in regulating the breathing rhythm but by no means 

 the only ones. They are, however, the ones of most impor- 

 tance in the present connection. 



195. Gaseous exchange through the skin. In addition to 

 the exchange of gases between the air and the blood which 

 goes on in the lungs, a similar process takes place, though to a 

 much smaller extent, through the skin. The true skin, under- 

 lying the cuticle or scarf-skin, is penetrated by capillary blood 

 vessels, and in its passage through these capillaries the blood 

 gives off some carbon dioxid and takes up some oxygen by dif- 

 fusion through the skin. The amounts given off and taken 

 up are small compared with the corresponding amounts in the 

 lungs, but still are not inconsiderable, and must be taken into 

 account in accurate experimental work. 



3. EXCRETION 



196. Excretory products. As already implied, the vital 

 activities of the body cells lead to the formation of products 

 which must be removed from the cells and some of which must 

 ultimately be discharged from the body. The next chapter 

 will be concerned with the nature of the more important of 

 these products and with some of the steps by which they are 

 formed. For the present, it suffices to say that the gradual 

 oxidations of non-nitrogenous material taking place in the cells 

 give rise substantially to the production of carbon dioxid and 

 water, while the proteins and related substances yield in addition 

 certain comparatively simple nitrogenous substances of which 

 the most abundant is urea. In addition to these substances, 

 more or less of the mineral ingredients also pass into the excreta. 



197. Excretion of carbon dioxid. As stated in the previous 

 section, the carbon dioxid produced by the tissue respiration 

 passes by way of the lymph into the blood and is excreted 

 through the lungs and to a minor degree through the skin. In 

 the blood the carbon dioxid is carried by both the corpuscles 

 and the plasma, but chiefly (two-thirds or more) by the latter, 

 in combination with proteins and haemoglobins, but especially 



