248 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



The hepatic portal system is similar to that hi the forms already discussed. The renal 

 portal system is identical with that of reptiles. The inferior mesenteric vein which connects 

 the two portal systems is probably homologous with the ventral abdominal veins of reptiles. 

 It is, however, of decreased importance as a channel between the two portal systems, owing 

 to the junction of the renal portal system with the postcaval vein. 



F. THE CIRCULATORY SYSTEM OF THE MAMMAL 



The specimen should have been injected in the arterial system. 



i. The chambers of the heart. The heart is relatively large and compact, 

 the chambers closely united with each other. The pericardial sac should be 

 removed if this has not been done previously. In case the thymus gland is 

 well developed, it will be necessary to dissect this away from the anterior part 

 of the heart. The greater portion of the heart consists of the two ventricles. 

 These constitute a firm thick-walled cone, having a posterior pointed apex and a 

 broad anterior base. This cone consists of two completely separated ventricles, 

 the right and left ventricles; the division between them is marked externally 

 by an indistinct line or groove extending from the left side of the base obliquely 

 to the right, and terminating to the right of the apex. The groove contains 

 branches of the coronary artery and vein which will be found ramifying over the 

 surface of the ventricles. The left ventricle is much the larger of the two and 

 includes the apex. Anterior to the base of each ventricle is a much smaller, 

 thin-walled, generally dark-colored chamber, the auricle or atrium. Each 

 auricle in the contracted state presents a lobe, the auricular appendage, project- 

 ing medially and slightly posteriorly over the ventricle ; in the cat (and man) 

 this lobe has a scalloped margin and is shaped something like the human ear 

 (hence the name auricle, meaning little ear). 1 Extending anteriorly from the 

 middle of the base of the ventricles forward between the two auricles is a large 

 artery, the pulmonary artery. This makes an arch to the left and disappears. 

 Dorsal to the pulmonary is another arterial trunk, the aorta. These two trunks 

 are generally imbedded in fat which should be removed. They represent the 

 split ventral aorta. A conus arteriosus is lacking, the arteries springing directly 

 from the ventricles. Grasp the apex of the heart and turn the heart forward 

 and note the bases of the great veins (pulmonary and systemic veins) entering 

 the auricles. There is no sinus venosus present as a distinct chamber; it is 

 greatly reduced and absorbed into the right auricle where it may be located by 

 physiological experiments as a small spot at the point where the systemic veins 

 enter the right auricle. From the morphological point of view, we may say that 

 the mammalian heart consists of but two different chambers (each of which is, 

 however, double) in contrast with the fish and amphibian heart, in which there 



1 In human anatomy only the ear-shaped lobe is named the auricle, the whole chamber being 

 termed atrium. In comparative anatomy, however, the terms atrium and auricle are regarded as 

 synonymous. 



