THE FROG in 



on each side of the vein, leaving it attached to a narrow strip. Free 

 the attachments of the liver 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, the liver, and other internal 

 organs. If the animal is a female, the ovaries should be removed 

 in case they obscure the other organs. 



The heart and the blood vessels leaving and entering it have 

 already been studied (page 98). 



The Veins. 1 The veins may be divided into two groups, which 

 are (1) 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 : ( 1 ) the caval veins, which bring blood 

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

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



Three large caval veins are present, which enter the sinus 

 venosus; two of these, the right and left precavals, bring blood 

 from the anterior half of the body, including the forelegs; the 

 other, the postcaval, brings blood from the posterior half of the 

 body. The blood brought by the postcaval vein is venous blood, 

 but with that brought by the precavals a certain quantity of 

 arterial blood has mingled. The prominent cutaneous veins which 

 we have already observed on the inner surface of the skin belong to 

 the precaval system ; and as the blood in them has been purified by 

 contact with the air through the skin and the mucous membrane 

 of the mouth and pharynx, that poured by the precavals into the 

 heart is a mixed blood. 



Turn the apex of the heart forward and observe the two 

 large precaval veins which enter the sinus venosus at its forward 



1 The veins are usually easily studied without being injected, as they are col- 

 ored 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 system, and through the postcaval vein for the remaining systemic veins. 

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

 through the ventricle and truncus arteriosus. 



