A FROG i8i 



driving the blood to all parts of the body ; (2) the arteries, the 

 vessels through which the blood is carried away from the heart ; 

 (3) the veins, the vessels through which the blood is returned to 

 the heart ; (4) the capillaries, the minute vessels which connect 

 the veins and arteries. 



Kill a frog and pin it down as directed on page 167. Make a 

 midventral incision through the skin from the tip of the snout 

 to the anus. Note the prominent cutaneous veins on the inner 

 surface of the skin. Identify first the abdominal vein through 

 the ventral wall of the abdomen. This vein lies in the body 

 cavity against the ventral abdominal wall and will appear as a 

 dark median line. Open the body cavity by a longitudinal in- 

 cision in the midventral line from the anus to the tip of the 

 lower jaw. Take great care not to cut the organs within. 



Free the attachments of the Kver to the body wall, spread the 

 two flaps to the right and left, making a short transverse cut in 

 each, and pin them fast, exposing fully the heart, liver, and other 

 internal organs. In the female animal the ovaries should be 

 removed if they obscure the other organs. 



The heart and the blood vessels leaving and entering it have 

 already been studied (page 172). 



The Veins.^ The veins may be divided into two groups, which 

 are (i) the systemic veins, those which enter the sinus venosus, 

 with their branches, bringing for the most part venous blood 

 from the various organs and tissues; and (2) the pulmonary 

 veins, which enter the left auricle, bringing arterial blood from 

 the lungs. 



We shall first study the systemic veins. These may be sub- 

 divided into two groups : (i) the cava! veins, which bring blood 

 directly to the heart ; (2) the portal veins, which bring blood 

 directly to the liver and kidneys, whence it goes to the heart. 



1 The veins and larger arteries are usually easily studied without being injected, 

 the veins being colored by the blood in them. The animals should not be dissected 

 fresh, but the blood should be permitted to harden in the veins first. If it is wished 

 to inject them, this should be done through the abdominal vein in both directions 

 for the portal systems, and through the postcaval for the remaining systemic 

 veins. The arteries can be better studied if they are injected ; this should be done 

 through the ventricle and truncus arteriosus. 



