56 GENERAL BIOLOGY 



have their own supply of blood to feed the liver and kidney substance, 

 in addition to that received from their respective portal veins. 



All the blood coming from the heart, and passing directly back to 

 the heart, whether it flows through the portal, renal-portal, abdominal or 

 other veins, is classified as the systemic circulation. This is to be dis- 

 tinguished from the pulmonary circulation, which deals with the blood 

 which, having been returned by the veins to the heart, is now sent to 

 the lungs to be purified and aerated. This blood leaves the heart ven- 

 tricle through the pulmonary arteries and is returned to the heart auricle 

 through the pulmonary vein. 



It is interesting to note, that in the frog, a part of the already-used- 

 blood (venous blood) which in the human being all goes to the lung 

 through the pulmonary artery, passes through the cutaneous artery, a 

 branch of the pulmonary, to the skin under the arm, where it is also 

 purified by the oxygen in the water. It will be remembered that the 

 frog needs both air and water for breathing purposes, and breathes 

 through both lungs and skin. 



The frog's heart is composed of three compartments (Fig. 10), in- 

 stead of four, as in the higher forms of animals. The blood that has 

 been purified in the lungs flows into the left auricle through the pul- 

 monary vein and is thus kept separate from the impure blood in the right 

 auricle ; x but, as there is only one ventricle, and as blood is always re- 

 ceived by the auricles, and always expelled from a ventricle, the impure 

 blood from the right auricle, as well as the pure blood from the left 

 auricle, is all emptied into one ventricle so that it is bound to intermingle. 

 However, the blood from the right side is a little more impure than on 

 the left, because the left side is directly connected with the left auricle 

 and it is the left auricle which has the purest blood. The pure and im- 

 pure blood are also kept partly separated by various irregular muscular 

 partitions called trabeculae extending through the ventricle. 



The action of the heart is as follows : The two auricles filled with 

 blood contract at the same time, thus forcing arterial and venous blood 

 into the ventricle. Here the two kinds of blood are kept from mixing 

 by the muscular trabeculae just mentioned. At the systole of the ven- 

 tricle, the venous blood, which lies nearest the bulbus arteriosus, is first 

 forced forward. This takes the most direct course through the wide and 

 short pulmonary arteries which are practically empty at the time. The 

 mixed arterial and venous blood follows the next easiest course through 

 the aortic arches, while the last blood to leave the. ventricle (the pure 

 arterial blood), can only go to the carotids, where the resistance is 

 greater on account of the small size of the vessels and the obstacles pre- 

 sented by the carotid glands. 



Blood usually looks red. This is due to the large red corpuscles 

 (Gr. erythrocytes=rerythros red+cytos=cell). The redness itself Js 

 due to what is called haemoglobin, a chemical substance contained within 



