PREGNANCY AND PARTURITION 1285 



or absent. The alimentary apparatus is for the most part ready. Thus 

 pepsin can be extracted from the festal gastric mucous membrane. The 

 pancreas contains trypsinogen and the intestinal mucous membrane pro- 

 secretin. Amylolytic ferments seem however to be absent both from 

 the salivary glands and the pancreas. The liver stores up glycogen and 

 secretes bile, which accumulates in the small intestine, forming the meco- 

 nium. This is generally voided by the child shortly after birth. 



THE FCETAL CIRCULATION. In the foetus, from the middle of intra- 

 uterine life, we find certain arrangements of the circulation which are 

 directed to providing the forepart of the body, especially the rapidly growing 

 brain, with oxygenated blood, while the less important tissues of the limbs 

 and trunk receive venous blood (Fig. 578). The arterial blood coming from 

 the placenta along the umbilical vein can pass directly into the liver. The 

 greater part of it however traverses the ductus venosus to enter the inferior 

 vena cava, by which it is carried to the right auricle. Here it impinges on 

 the Eustachian valve, and is directed thereby through the foramen ovale into 

 the left auricle, whence it passes into the left ventricle to be driven into the 

 aorta. As this arterial blood passes into the inferior cava, it is of course 

 mixed with the venous blood, returning from the lower limbs and lower part 

 of the trunk. By the aorta this mixture, containing chiefly arterial blood, 

 is carried to the head and fore limbs. The venous blood from these parts is 

 carried by the superior vena cava to the right auricle, and thence to the 

 right ventricle, by which it is driven into the pulmonary artery. Only a 

 small part of the blood passes through the lungs, the greater part 

 traversing the patent ductus drteriosus to be discharged into the aorta 

 below the arch, whence it flows partly to the lower limbs and trunk, but 

 chiefly to the placenta by the umbilical arteries. In the foetus therefore 

 the work of the circulation is largely carried out by the right ventricle. The 

 greater thickness of the left ventricular walls, which is so characteristic of 

 the adult, does not become evident until shortly before birth. 



With the first breath taken by the newborn child all the mechanical 

 conditions of the circulation are modified. The resistance to the blood flow 

 through the lungs being diminished, the blood passes from the pulmonary 

 arteries through the lungs into the left auricle. The pressure in the left 

 auricle is thus raised, while that in the right auricle falls, so that the foramen 

 ovale is maintained closed. Even before birth proliferation of the lining 

 membrane may be seen both in the ductus arteriosus and in the ductus 

 venosus ; and with the mechanical relief of the vessels afforded by respira- 

 tion and the changed conditions of the foetus, this proliferation goes on to 

 complete obliteration of the vessels. 



PARTURITION 



As the uterus increases in size and becomes more distended, its irritability 

 becomes greater, so that it is easily excited to contract. The stimulus may 

 be supplied from adjacent abdominal organs, from the brain, as by emotions, 

 or by direct excitation of the internal surface of the uterus, in consequence 



