56 General Biology 



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 

 distinguished 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 

 ventricle 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), 

 instead of four, as in the higher forms of animals. The blood, which 

 has been purified in the lungs, flows into the left auricle through the 

 pulmonary vein and is thus kept separate from the impure blood in the 

 right auricle. But, as there is only one ventricle, and as blood is always 

 received 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 

 impure 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 the arterial blood from 

 the left auricle and the venous blood from the right auricle into the 

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

 trabeculae just mentioned. At the systole of the ventricle, some of the 

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

 forward. This blood takes the most direct route through the wide and 

 short pulmocutaneous arteries which are practically empty at the time. 

 Some of the arterial blood is next forced out through the carotid arches 

 to the head region, while the last blood to leave the ventricle is a mixture 

 of the remaining arterial and venous blood, which is forced through the 

 systematic arches to supply the general body system. 



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

 (Gr. erythrocytes=erythros — red+cytos=cell). The redness itself is 

 due to haemoglobin, a chemical substance contained within the cor- 

 puscles. There are also white corpuscles, called leucocytes (Gr. leukos= 



