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VASCULAE SYSTEM 



septum joins the fused cushions these openings lead from the two auricles, but 

 at first they both communicate with the left side of the ventricular chamber. 

 Tn consequence, however, of the rotation above described, the canal is moved 

 towards the right ; and when the septum of the ventricles, presently to be described 





FIG. 268. SECTION OF THE TBUNK OF A HUMAN EMBBYO OF 15'5 MM. (T. H. Bryce.) 



The section cuts the pericardium, heart, and lungs. Below the neural canal is the centrum of a 

 vertebra, which is connected with the neural arch on either side, and with a pair of ribs. To the inside 

 of the cartilage of the neural arch are seen the spinal ganglia ; outside are the myotomes separated into 

 dorsal and ventral portions by the dorsal branches of the spinal nerves. Below the vertebra is the aorta 

 giving off a pair of segmental arteries. On each side of the aorta the cardinal veins, and farther out 

 the sympathetic cords. Below the aorta the oesophagus and trachea ; on each side the lungs ; the 

 letters rl and vl are placed in the pleural sacs ; notice how they are separated from the ribs by a thick 

 lamella of very open connective tissue. The pleural spaces are separated from the pericardium P by 

 the pleuro-pericardial membranes. The heart is attached to the mesenteric septum by themesocardium. 

 Attached to this on the left side by a fold is the now much reduced left duct of Cuvier. r.v. right, 

 Lv. left ventricle ; r.a. right, La. left auricle ; s.p., septum primum ; the arrow marks the ostium 

 secundum ; r.v.v., right venous valve. Above it is the opening of the sinus venosus, closed on the left 

 by the left venous valve. 



also unites with the fused endocardial cushions, they come to be connected each 

 with its proper ventricle. The formation of the auriculo-ventricular valves will be 

 considered in the following paragraph. 



Ventricular chamber. The common ventricular chamber is formed, as 

 already stated from the descending limb and transverse portion of the ventricular 



