26 THE FROG 
into three branches; trace their distribution. What evidence do 
you find that the skin of the frog is an important organ of 
respiration? 
D. The Capillaries, and their Relation to Arteries and Veins.— 
Examine a microscopic demonstration of the circulation of the 
blood in the web of the frog’s foot. Arteries may be distin- 
guished 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 unin- 
jected 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 figure 
75 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 vene cave, the posterior vena cava, 
and the pulmonary vein. The anterior vene cave receive blood 
from the head, arms, and anterior part of the body generally; 
the posterior vena cava receives blood from the viscera (exclud- 
ing the lungs) and the posterior part of the body; the pulmonary 
vein, through its two branches, receives blood from the lungs. 
1. Vessels which enter the anterior vene cave. 
(a) The external jugular entering anteriorly is formed by the 
union of the lingual vein, conveying blood from the floor of the 
mouth and the tongue, and the mandibular vein, running ventral 
to the lower jaw. 5 
(b) The innominate vein entering laterally is formed by the 
union of the internal jugular, conveying blood from the interior 
of the skull, and the subscapular, conveying blood from the dorsal 
side of the arm and the shoulder. 
(c) The subclavian vein, entering laterally and posteriorly, is 
formed by the union of the brachial vein, returning blood from 
