THE CCELOM. 71 



extremity, and by its right common carotid branch to the right side of the head 

 and neck ; another part enters the left common carotid artery and is distributed to 

 the left side of the head and neck, and some passes, through the left subciavian 

 artery, to the left upper limb. The remainder mixes with the blood which enters 

 the aorta, from the right ventricle, through the pulmonary artery and the ductus 

 arteriosus. Part of this mixed blood is distributed to the body and the viscera, 

 and the lower limbs, and the remainder passes through the umbilical arteries to the 

 placenta (Fig. 88). 



The Foetal Circulation. When the fcetal circulation is thus fully established, 

 purified oxygenated blood, returning from the placenta, enters the body of the 

 foetus by the umbilical vein and passes to the liver. Some of it enters the liver, 

 but the greater part passes, through a channel called the ductus venosus, to the 

 inferior vena cava, where it mixes with the venous blood returning from the 

 lower limbs and the abdominal region, including the liver. This mixed, but, 

 as contrasted with the blood in the superior vena cava, comparatively pure blood 

 enters the right atrium and passes through it and through the foramen ovale into 

 the left atrium, thence to the left ventricle and through the left ventricle into 

 the aorta. A portion of this comparatively pure blood is distributed to the head 

 and neck and the upper limbs. The remainder unites with the stream of venous 

 blood poured into the aorta through the ductus arteriosus. Part of it is distributed 

 to the body and the lower limbs, and part is sent to the placenta to be purified 

 and oxygenated (Fig. 88). 



The remaining part of the blood stream is formed by the blood returned from 

 the head and neck, the upper part of the body and the upper limbs, by the 

 superior vena cava, and from the walls of the heart by the coronary sinus. It 

 is the most venous and impure blood in the body. After entering the right 

 atrium it passes into the right ventricle, and thence into the pulmonary artery. 

 A very small part of it is passed to the lungs, by the right and left branches 

 of the pulmonary artery ; the remainder goes through the ductus arteriosus into 

 the aorta, where, beyond the origin of the left subciavian artery, it mixes with the 

 much purer blood which entered the aorta from the left ventricle. 



At birth; when the placental circulation ceases, the lungs become the organs 

 through which oxygen enters and carbonic acid leaves the blood; the foramen 

 ovale in the interatrial septum closes, and the ductus arteriosus is obliterated. 

 The course of the circulation and the condition of the blood in the different regions 

 is, therefore, considerably altered. 



On account of the cessation of the placental circulation all the blood which 

 enters the right atrium is entirely venous, and, as the foramen ovale is closed, 

 it all passes into the right ventricle, which forces it into the pulmonary artery. 

 As the ductus arteriosus is closed, all the blood which enters the pulmonary 

 artery must now pass through the lungs, where it is aerated, and whence it is 

 returned, by the pulmonary veins, as oxygenated blood, to the left atrium. It 

 passes from the left atrium to the left ventricle, which forces it through the 

 aorta and its branches to all parts of the head, neck, body, and limbs; and 

 now, for the first time, all parts receive blood of the same quality. 



THE CCELOM. 



It has already been pointed out that there are two parts of the ccelom, the 

 extra-embryonic and the intra-embryonic. Both are clefts separating an outer from 

 an inner layer of mesoderm. 



The Extra-embryonic Coeloxn. The extra-embryonic coelom appears in the 

 primary mesoderm and separates it into a parietal and a visceral layer. The 

 parietal layer covers the inner surface of the trophoblast and forms with it the chorion. 

 It covers also the outer surface of the auinion. The visceral layer covers the outer 

 surface of the extra-embryonic portion of the wall of the entodermal cavity. 



The extra-embryonic and intra-embryonic parts of the coslom are at first 

 saparate from one another (Fig. 36), then they become continuous, for a time, in the 

 region of the umbilical orifice (Fig. 37), but are separated from one another again 



56 



