282 STUDY OF PIG EMBRYOS. 



of the adult being formed from the left aorta. Lower down in the series the two 

 descending aortse unite to form the single median dorsal aorta. The common 

 cardinals, C.C.S, are two enormous venous trunks which deliver the blood to the 

 heart. They lie symmetrically placed to the right and left of the oesophagus and 

 trachea. They extend from the level of the descending aortae downward and inward 

 to the level of the heart. The vein of the left side, C.C.S, is almost symmetrical 

 with its fellow of the right side, though it has no direct communication with the 

 heart; but by following down through the series of sections the student can 

 observe that the left common cardinal connects across with the corresponding vein of 

 the right side. The right vein opens directly into the right auricle, Au.d, of the 

 heart. All of the venous blood is collected at this stage by the common cardinals, 

 except that which comes through the liver. The common cardinals are formed by 

 the union of the jugular or anterior cardinal vein from the head with the posterior 

 cardinal vein from the body. The opening of the right vein into the auricle of 

 the heart is guarded by two small flaps or valves. The lower part of the section 

 is occupied by the large heart lying in the pericardial chamber. The body-wall, 

 Som, or somatopleure, which forms the outer covering of this chamber, is quite thin 

 and without a trace of muscular or skeletal structures. It consists of three distinct 

 layers the external ectoderm, the middle mesenchyma, and the internal mesothelium. 

 The mesothelium is a thin layer of cells which persists throughout life and is 

 known in the adult as the pericardial epithelium. In the present section it is easy 

 to follow this layer from the somatopleure past the common cardinals on to the 

 heart and completely around the outside of the heajrt itself. Everywhere it forms 

 the covering or boundary of the ccelom of the pericardium. In later stages this 

 mesothelium will have an especial layer of connective tissue close under it.- The 

 layer of connective tissue, together with the mesothelium, constitutes the pericardial 

 membrane of descriptive anatomy. The essential fundamental relations - of this 

 membrane may, therefore, be easily understood from the present section. From 

 the study of the adult conditions alone it is extremely difficult for the student to 

 grasp these relations. The heart is a very large organ. It consists of two auricles 

 and a ventricle with two limbs. The auricles have thin walls and are separated 

 from one another by a very thin membrane, the septum superius, S.s. The right 

 auricle, Au.d, receives upon its dorsal side the opening of the right vein or common 

 cardinal, the opening being guarded by valves. Of these valves, the one toward 

 the median line disappears, but the other, toward the right of the embryo, per- 

 sists to form both the Eustachian and Thebesian valves of the adult. As stated 

 above; the left common cardinal delivers its blood to the right vein, and so indi- 

 ^eart. The ventricles of the heart are much larger than the auricles, 

 icular limb or future left ventricle, Ven.S, is already larger than 

 external groove, F, which marks the boundary between the 

 ^vn by the section. The trabecular structure of the 

 '"fords a diagnostic n^ark by which the ventricles. 



