THE FCETUS. 



1045 



Instead of going to the lungs — which do not yet act as respiratory organs — the 

 blood, being pressed by the contraction of the right ventricle, passes into the 

 aorta by the ductus arteriosus. 



To sam up, the foetus never receives pure arterial blood into its organs, this 

 being always mixed with venous biood — the mixture taking place at several 

 points : 1. By the foramen of Botal. 2. In the aorta by the ductus arteriosus. 

 3. In the liver by tlie ductus venosus. The head and neck are the parts which 

 receive the purest arterial blood — a fact that explains the predominance of the 

 anterior over the posterior portion of the body of the foetus. 



At birth, the conditions of existence being suddenly changed, marked modifi- 

 cations take place in the circu- 

 latory apparatus. The lungs ^'S- ^^l- 

 become the organs of respira- 

 tion, and rapidly increase in 

 capacity ; the pulmonary artery 

 dilates to give passage to the 

 blood that flows to them ; 

 while the ductus arteriosus is 

 obliterated, in order to isolate 

 the arterial from the venous 

 blood. This separation of the 

 two fluids also takes place in 

 the liver by the atrophy of the 

 ductus venosus, and in the 

 heart by the occlusion of the 

 foramen of Botal ; though, 

 according to Goubaux, that 

 orifice frequently remains open 

 in young animals. Its persis- 

 tence has also been noted in 

 the human adult. Notwith- 

 standing the presence of this 

 foramen, the circulation can- 

 not be much disturbed ; as 

 when the heart contracts, the 



auricles become isolated by the constriction of the opening and the raising of a 

 valve. 



(Lymphatic vessels and glands are found at an early period of foetal life, after 

 the blood-circulation has been fully established. In connection with these 

 should be mentioned the prevertebral hmno-lymph glands, which are best observed, 

 however, in the adult animal — and particularly in the Sheep and Ox. They are 

 small oval or lenticular bodies, about the size of a mustard-seed, and are found 

 in the abdominal and pelvic cavities among the fat and connective tissue, between 

 the vertebral column and peritoneum, as well as in the thorax, in the middle and 

 posterior mediastinum. They are larger in Oxen than Sheep ; there are from 

 three to four hundred in the latter. They are supposed to be the source of 

 some, at least, of the lymphoid cells and non-nucleated and multi-nucleated 

 corpuscles which occur in the blood.) 



TRANSVERSE SECTION OF THE ADVANCED EMBRYO OF A 

 RABBIT, SHOWING THE RELATIONS OF THE HEART WITH 

 THE LUNGS AND DIGESTIVE TUBE. 



ht, heart ; pc, pericardial cavity ; pl.p, pleural cavity ; Ig, 

 lungs ; al, digestive canal ; ao, aorta ; ch, notochord ; rp, 

 ribs; st, sternum; sp.c, spinal cord. 



