SYSTEM. 



called Ostium V^enosum, communicating from the auricles to 

 the ventricles, and which is afterwards divided into two, one 

 for either side of the heart. The partition between the ventri- 

 cles is completed about the end of the second month of gestation, 

 at a period when the aorta, from having been simple original- 

 ly, is converted into two canals, one of which becomes the pul- 

 monary artery. The partition between the auricles is not com- 

 pleted till birth. In cases of monstrosity, it is interesting to see 

 how much the heart, at the *end of uterine life, has still pre- 

 served this original type of simplicity. I have lately dissected 

 a double foetus, where, from the parasitical character of one, 

 no effort had been made for the development of the lungs of the 

 latter. The consequence of which was, the parasite's heart 

 consisted only of the right auricle and of the left ventricle, and 

 the pulmonary artery had not been formed at all, there being but 

 the single tube, the aorta, which led from the left ventricle, and 

 had a sort of arrangement in its branches depending upon the 

 tendency to form pulmonary arteries.* 



At birth, the auricular septum has advanced so far that the 

 communication between the two cavities is kept up only by a 

 deficiency, called the Foramen Ovale. This foramen, marked 

 by a depression on the right side, admits a small quill, when 

 conducted obliquely through it, and is protected on the left side 

 by a valve, the edge of which is upwards, and which, when ap- 

 plied, is just large enough to cover the whole foramen. The 

 moment that the blood ceases to pass through the foramen 

 ovale, which occurs at the first act of inspiration, the valve is 

 applied, and the aperture grows up by the adhesion of its 

 edge. The mechanism of this process is sufficiently simple. 

 So long as the principal current of the blood was into the right 

 auricle, the valve was pushed off from the side of the septum; 

 but as breathing establishes, through the lungs, pulmonary veins, 

 and left auricle, a current of circulation equivalent, both in 

 quantity and force, to that through the two vena3 cavae and 

 right auricle, a perfect equilibrium between the auricles is esta- 

 blished, and the valve retains its place against the septum. 

 Notwithstanding the incessant action of the auricles, during all 



* For a detail of this case, s^e North American Medical and Surgical Journal, 

 Philad. Oct 1826. 



