RESPIRATION. 71 



THE LUNGS. This necessary organ of respiration is of 

 a spongy texture, lodged in the thoracic cavity, divided into 

 two independent halves or lobes a right and a left, the 

 left being a little smaller than the right lobe. The pul- 

 monary tissue of the mature animal is of a bright rose 

 color ; in the foetus its color is deeper because not yet in- 

 flated with air. The tissue is soft but very strong and 

 remarkably elastic. It is very light, floats in water if 

 healthy, and this is attributed to the air held in the lung 

 vesicles. The lung of a foetus will sink in water, but after 

 once being inflated, the air cannot be expelled so as to 

 cause it to sink. The relative weight of the lungs to body 

 is much greater in the adult animal than in the foetus, it 

 being one-thirtieth in the former to one-sixtieth of the 

 whole body in the latter. 



It is demonstrated that the blood, after losing its bright 

 red color and the properties which maintain the vitality of 

 the tissues, returns from all parts of the body by the veins 

 to the right side of the heart, and is propelled thence into 

 the lung where it is regenerated by contact with the air. 

 These air cells or vesicles in the lungs are wonderfully mi- 

 nute, being only from 1-3800 to 1-1600 part of an inch in 

 diameter. And between these vesicles is an exceedingly 

 thin, elastic tissue, with a few muscular fibres. The pul- 

 monary veins carries the blood back to the heart after re- 

 generation in the lungs. The principal thing to remember 

 is, that the lung is the seat of the absorption of oxygen by 

 and the expulsion of carbonic acid from the returned or 

 vitiated blood, or the transformation of dark into bright 

 red colored blood. 



The lung is early developed in the foetus, and its lobular 

 texture is well defined through the whole period of foetal 

 existence. 



Respiratory Action of the Skin. The skin is the seat of 

 a constant and important respiratory action, as it absorbs 



