THE THORAX OF THE FffiTUS. 531 



in adults, according to Biot, in the ratio of one to four. But 

 judging from my own observations, the proportion of cases in 

 which the opening exists is by no means so great. Someftimes 

 the foramen is met with in adults so dilated as to be nearly an 

 inch in diameter. I have met with two cases of this sort in the 

 dissecting-room, both of which occurred in females between 

 twenty and thirty years of age. The nutritive functions 

 appeared to have been perfectly well performed in both these 

 subjects, judging from the state of the body ; the right auricle 

 and ventricle were dilated and hypertrophied so as to present 

 the same thickness of parietes as the corresponding parts of the 

 left side. The tricuspid valves, and the semilunar valves of the 

 pulmonary artery were thickened, and presented cartilaginous 

 concretions on their edges, in which the work of ossification had 

 just commenced. This thickening and ossification of the valves 

 in the normal formation of the heart is almost wholly peculiar 

 to the valves of the left side, and appears to be caused, as was 

 first suggested by Cruveilhier, by the force with which the blood 

 is dashed against the valves, in the forcible contractions of the 

 ventricle. 



The, Pulmonary Artery and the Aorta, 



Have a communication in the foetus, which is very analogous 

 to the communication between the auricles of the heart. 



From the pulmonary artery, where it divides into the two 

 great branches, another large branch continues in the direction 

 of the main trunk, until it joins the aorta ; with which vessel it 

 communicates at a small distance below the origin of the left sub- 

 clavian artery. In the young subject that has never respired, 

 it appears as if the pulmonary artery was continued into the 

 aorta, and sent off in its course a branch on each side, much 

 smaller than itself, to each lung. In subjects that have lived 

 a few days, these branches to the lungs are much larger ; and 

 then the main pulmonary artery appears to have divided into 

 three branches : one to each lung, and one to the aorta ; but 

 that which continues to the aorta is larger than either of the others. 



