372 



THE FROG. PHYLUM CHORDATA 



aU 



irJ 



■cap 



F"iG. 289. — Capillaries in the 

 web of a frog's foot. — 

 After Allen Thomson. 



1?/., Arteriole; cap., capillaries; 

 vnl., venule. 



walls so that they can withstand the high pressure caused by the 

 heart-boat, and after many subdivisions become small vessels 

 known as arterioles. These in turn divide into minute, very thin- 

 wallod vessels called capillaries (Fig. 289), that 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 

 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 (Fig. 290). Through heart, arteries, 

 capillaries, 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 or in the tail of a 

 tadpole. In the arteries the blood flows fast 

 and with jerks, which are caused by the beat- 

 ing of the heart and known as the pulse. 

 In the arterioles the increased friction owing 

 to the increased surface 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. 



-< a?;?:?r .. 



Jl a 



Fig. 290. — Diagrams 

 of a valve in a vein 



A, Part of the wall of the 

 vein from within ; B, 

 longitudinal section of 

 the vein, showing the 

 position of the valve 

 when blood flows from 

 the direction of the 

 capillaries (c) towards 

 the heart (h) ; C, similar 

 section showing how 

 reflux is prevented by 

 the valve. 



LYMPH 



The blood vessels are completely closed, and the tissues are 

 actually bathed by fluid, known as lymph, w^hich is that part of 

 the blood which is able to escape from the vessels (p. 525). This 

 fluid is gathered by small lymphatic vessels into the big lymph 

 sacs already mentioned, whence it is pumped back into the veins 



