THE FROG 39 



means of gills? How does the arterial circulation of a gill- 

 breathing amphibian (e. g., Necturus, or the frog tadpole) 

 compare with that of a fish (examine preparations of the 

 arterial system of the latter) ? 



D. The Capillaries, and their Relation to Arteries and 

 Veins. Examine a microscopic demonstration of the cir- 

 culation of the blood in the web of the frog's foot. Arteries 

 may be distinguished from veins by the fact that the blood 

 corpuscles scatter to enter the capillaries diverging from an 

 artery, while in the veins the corpuscles accumulate from the 

 capillaries converging to form the vein. A slight pulsation 

 may sometimes be observed in the smaller arteries. In the 

 capillaries the corpuscles usually advance in single file. 



E. The Veins. For the following study use an entire un- 

 injected specimen; if possible leave the specimen in such 

 condition that it may be used for the study of the veins by 

 future classes. The veins may be distinguished from the 

 arteries by the fact that the former are usually filled with 

 dark-colored blood. As an aid to the identification of the 

 veins the student may consult Leuckart's chart, or Figs. 75 

 and 77 of Holmes' Biology of the Frog. A drawing is not 

 required. 



The veins convey blood from the capillaries toward the 

 heart. We have seen that the blood reaches the heart 

 through four vessels, viz., two anterior venae cavse, the 

 posterior vena cava, and the pulmonary vein. The anterior 

 venae cav?e receive blood from the head, arms, and anterior 

 part of the body generally; the posterior vena cava receives 

 blood from the viscera (excluding the lungs) and the pos- 

 terior part of the body; the pulmonary vein, through its two 

 branches, receives blood from the lungs. 



