52 ZOOLOGY. 



Description of the Circulation of the Blood in the Higher 

 Animals. 



92. The Heart. In man, and all animals similarly or- 

 ganized, the heart is lodged "between the lungs and in the 

 cavity of the chest called by anatomists the thorax (Figs. 5 

 and 58) : its lower extremity is directed somewhat obliquely 

 towards the left side and forwards ; and its upper extremity, 

 from which spring the great vessels, is fixed to the neigh- 



vj ac i ac vj 



T> . . T 

 Eight Lung. 



od vc iid a vg 



Fig. 39. Lungs, Heart, and Principal Vessels in Man.* 



bouring parts, nearly in the mesial plane of the body. 

 Throughout the rest of its extent the heart is free, but is 

 surrounded by a fibro- serous membrane, called the pericar- 

 dium, which not merely forms a shut sac for containing the 

 heart, but gives to its surface a smooth covering, by which it 



* od, vd, right auricle and ventricle; vg, left ventricle; a, aorta; 

 ac, carotid arteries ; vc, vena cava inferior j vj, jugular veins j 

 t % trachea. 



