CONGENITAL MALFORMATIONS. 299 



and after folding it lengthwise, twist it forcibly from left to 

 right; then, holding it vertically, let us try and approximate the 

 two ends which we hold — one in each hand ; the immediate 

 result is a loop, with its convexity turned to the left. Now the 

 cardiac tube behaves exactly like this handkerchief, when the 

 aortic bulb approaches the sinus venosus. The loop turned to the 

 left corresponds to the heart's apex, while the piece below it 

 represents the inferior cava, that above it, the aorta. Tlie 

 cardiac tube therefore bends, not like a smooth cylinder, but like 

 one which has been twisted from left to right. We may there- 

 fore assume that in most persons the column of blood is twisted 

 spirally from left to right, and that to this cause is due the 

 usual position of the heart on the left side. 



But there are cases in which this rotatory movement takes 

 place in the opposite direction. In these, the curvature of the 

 cardiac tube corresponds to that of a cylinder which has been 

 twisted from right to left ; the heart's apex is directed to the 

 right side, and the asymmetrical viscera, whose development is 

 subsequent in point of time to that of the heart, are completely 

 transposed (situs viscerum inversus). The liver lies on the left 

 side, the spleen on the right ; the cardia is turned to the right, 

 the pylorus to the left ; the right lung has two, the left lung 

 three lobes, and so on. We may infer from this, that the asvm- 

 metry of the heart is responsible for whatever asymmetry exists 

 throughout the animal organism. 



§ 2G3. A new phase in the development of the heart is 

 brought to a close by the downward displacement of the common 

 pulmonary trunk along the anterior surface of the bullnis aorta', 

 and the simultaneous development of the septum vcntriculorum. 

 This process is also liable to be disturbed. For it sometimes 

 happens that the trunk of the pulmonary artery descends along 

 the posterior instead of the anterior surfiico of the aorta. We 

 thus find the aorta placed over the right ventricle, the j^nlnionar}/ 

 artery over the left one, a malformation which can only consist 

 with life so long as the circulation of Sabaticr remains open,- and 

 enables the pulmonary function to be dispensed with. 



§ 264. Finally, we have yet to consider congenital stenosis of 

 the right heart, and particularly of i\\Q conus arteriosus and the 

 orifice of the pulmonary artery. This is probably a result of 

 endocarditis during foetal life. We find just the same state of 



