THE HEART AND RESPIRATORY ORGANS. 9 



as to allow the food to be returned with ease into the mouth. The intes- 

 tines vary in length considerably. In the Carnivora, or flesh-caters, their 

 length is only three or four times the length of the animal's body ; in the 

 Hirbivora, or grass-eaters, they are from twelve to twenty-eight times 

 their length. It is worthy of notice, as indicating how a change in struc- 

 ture may be developed by change of food and habits, that the intestines 

 of the common cat whom domestic life has accustomed to a less carniv- 

 orous diet, are longer than those of its wild and bloody kindred. 



The heart of the Mammalia is a double heart, consisting of two 

 auricles and two ventricles, which are provided with valves so arranged 

 that the blood can flow from the auricle into the ventricle, but not from 

 the ventricle to the auricle. The course of the blood through this organ 

 may be briefly described. The venous blood that has become impure 

 in the tissues is returned by them to the right auricle, and is then dis- 

 charged into the right ventricle. The powerful muscles of the heart 

 thence propel it to the lungs, where it meets the air taken in by respira- 

 tion and is changed into arterial blood. From the lungs it is drawn back 

 into the left auricle, passes into the left ventricle, and is thence forced 

 through the arteries to all parts of the body, and then by means of the 

 exceedingly fine vessels called capillaries, passes again once more into 

 the veins. 



Venous blood is changed into arterial blood in the lungs, where it 

 absorbs the oxygen of the air. The lungs are two in number, one on 

 each side of the chest, and communicate by the bronchial tubes and the 

 trachea, or windpipe, with the outer air. The windpipe we can all feel 

 in the front of our necks ; it divides in the chest into the two bronchial 

 tubes, and they are subdivided into an infinite number of little rootlets 

 that enter into the substance of the lung. 



The air is taken into and discharged from the lungs by the operation 

 of breathing ; and breathing is effected by the elevation and depression 

 of the ribs and the contraction and relaxation of the flat, powerful mus- 

 cular partition which separates the cavity containing the stomach and 

 the intestines from the chest, which contains the heart and lungs. The 

 air we inhale enters the lungs laden with oxygen ; the air we exhale 

 leaves them laden with carbonic acid gas. 



The blood, which the lungs renew and the heart distributes, is of a 

 light red color. It is the substance which animates the whole being, and 

 from which all the complex structures of the body are formed. Blood 



