56 



MANUAL OF ELEMENTARY ZOOLOGY 



into the carotid comes only from the left auricle. The 

 meaning of this separation of the blood will be seen later. 

 To and from the heart leads a complicated system of 



blood vessels, through which the red blood is 

 th?Biood°. n0f driven by the heart-beat. The yessels_which 



lead from the heart are called arteries; those 

 which lead to the heart are veins. The arteries have thick, 

 muscular walls, and after many subdivisions become small 

 vessels known as arterioles. These in turn divide into 

 minute, very thin-walled vessels called capillaries, which lie 

 among the tissues in the form of a meshwork, which in 

 active tissues, such as glands and muscles, is exceedingly 

 fine, so that the blood is brought close to every part 

 From the capillaries the blood is collected into small 

 venules which join to form the veins. The walls of the 

 veins are thinner and less muscular but more elastic than 



those of the arteries, and 



all. 



vnl. 



many of them contain small 

 watch-pocket valves, placed 

 with the opening of the 

 pockets towards the heart 

 so as to prevent the blood 

 from being driven in the 

 wrong direction when the 

 vessels are compressed by 

 the movements of the body. 

 Through heart, arteries, cap- 

 illaries, and veins there 

 takes place a circulation of 

 the blood, which can be seen 

 under the microscope in the 

 capillaries of the thin web 

 between the toes of the frog's foot. In the arteries the 

 blood flows fast and with jerks, which are caused by the 

 beating of the heart and known as the pulse. In the 

 arterioles the increased friction owing to the increased sur- 

 face of the numerous branches obliterates the pulse. In 

 the capillaries the increased area lessens the rate of flow. 

 In the veins the blood is flowing back to the heart evenly 

 and less fast than in the arteries, though faster than in the 

 capillaries. 



— cap. 



FlG. 29. — Capillaries in the web 

 of a frog's foot.— After Allen 

 Thomson. 



all. , Arteriole ; cap., capillaries ; 

 vnl., venule. 



