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by veins which enter the heart. These vessels are known as the omphalomesenteric arteries and veins. The viteiline circulation is of short dura. tion in the mammals, as the supply of nutritious matter in the vitellus soon becomes exhausted.

The placental circulation becomes established when the blood-vessels in the allantois enter the villous processes of the chorion and come into close relationship with the maternal blood-vessels. This circulation lasts during the whole of intra-uterine life, but gives way at birth to the adult circulation, the change being made possible by the development of the circulatory apparatus.

The heart appears as a mass of cells coming off from the anterior por. tion of the intestine; its central part liquefies, and pulsations soon begin. The heart is at first tubular, receiving posteriorly the venous trunks and giving off anteriorly the arterial trunks. It soon becomes twisted upon itself, so that the two extremities lie upon the same plane.

The heart now consists of a single auricle and a single ventricle. A septum growing from the apex of the ventricle divides it into two cavities, a right and a left. The auricles also become partly separated by a septum which is perforated by the foramen ovale. The arterial trunk becomes separated by a partition into two canals, which become, ultimately, the aorta and pulmonary artery. The auricles are separated from the ventricles by incomplete septa, through which the blood passes into the ventricles.

Arteries.—The aorta arises from the cephalic extremity of the heart and divides into two branches, which ascend, one on each side of the intestine, and unite posteriorly to form the main aorta ; posteriorly to these first aortic arches, four others are developed, so that there are five altogether running along the visceral arches. The two anterior soon disappear. The third arch becomes the internal carotid and the external carotid; a part of the fourth arch, on the right side, becomes the subclavian artery, and the remainder atrophies and disappears, but on the left side it enlarges and becomes the permanent aorta ; the fifth arch becomes the pulmonary artery on the left side. The communication between the pulmonary artery and the aorta, the ductus arteriosus, disappears at an early period.

Veins.—The venous system appears first as two short, transverse veins, the canals of Cuvier, formed by the union of the vertebral veins and the cardinal veins, which empty into the auricle. The inferior vena cava is formed as the kidneys develop, by the union of the renal veins, which, in a short time, receive branches from the lower extremities. The subclavian veins join the jugular as the upper extremities develop. The heart descends in the thorax, and the canals of Cuvier become oblique; they shortly com

municate by a transverse duct, which ultimately becomes the left innominate vein. The left canal of Cuvier atrophies and becomes a fibrous cord. A transverse branch now appears, which carries the blood from the left cardiac vein into the right, and becomes the vena azygos minor; the right cardinal vein becomes the vena azygos major.

Circulation of Blood in the Fetus.—The blo returning from the placenta, after having received oxygen, and being freed from carbonic acid, is carried by the umbilical vein to the under surface of the liver; here a portion of it passes through the ductus venosus into the ascending vena cava, while the remainder flows through the liver and passes into the vena cava by the hepatic veins. When the blood is emptied into the right auricle, it is directed by the Eustachian valve through the foramen ovale, into the left auricle, thence into the left ventricle, and so into the aorta to all parts of the system. The venous blood returning from the head and upper extremities is emptied, by the superior vena cava, into the right auricle, from which it passes into the right ventricle, and thence into the pulmonary artery. Owing to the condition of the lung, only a small portion flows through the pulmonary capillaries, the greater part passing through the ductus arteriosus, which opens into the aorta at a point below the origin of the carotid and subclavian arteries. The mixed blood now passes down the aorta, to supply the lower extremities, but a portion of it is directed, by the hypogastric arteries, to the placenta, to be again oxygenated.

At birth, the placental circulation gives way to the circulation of the adult. As soon as the child begins to breathe, the lungs expand, blood flows freely through the pulmonary capillaries, and the ductus arteriosus begins to contract. The foramen ovale closes about the tenth day. The umbilical vein, the ductus venosus, and the hypogastric arteries become impervious in several days, and ultimately form rounded cords.

TABLE OF PHYSIOLOGICAL CONSTANTS.

Mean height of male, 5 feet 672 inches; of female, 5 feet 2 inches.
Mean weight of male, 145 pounds; of female, 121 pounds.
Number of chemical elements in the human body; from 16 to 18.
Number of proximate principles in the human body; about 100.
Amount of water in the body weighing 145 pounds; 108 pounds.
Amount of solids in the body weighing 145 pounds; 36 pounds.
Amount of food required daily; 16 ounces meat, 10 ounces of bread, 372

ounces of fat, 52 ounces of water.
Amount of saliva secreted in 24 hours ; about 372 pounds.

Function of saliva ; converts starch into glucose.

Active principle of saliva ; ptyalin.
Amount of gastric juice secreted in 24 hours ; from 8 to 14 pounds.

Functions of gastric juice ; converts albumin into albuminose.

Active principles of gastric juice; pepsin and hydrochloric acid. Duration of digestion; from 3 to 5 hours. Amount of intestinal juice secreted in 24 hours; about i pound.

Function of intestinal juice; converts starch into glucose. Amount of pancreatic juice secreted in 24 hours; about 1/2 pounds. Active principles of pancreatic juice; trypsin, amylopsin, and steapsin.

1. Emulsifies fats. Functions : 2. Converts albumin into albuminose.

3. Converts starch into glucose. Amount of bile poured into the intestines daily; about 272 pounds.

I. Assists in the emulsification of fats.

2. Stimulates the peristaltic movements. Functions :

3. Prevents putrefactive changes in the food.

4. Promotes the absorption of the fat. Amount of blood in the body; from 16 to 18 pounds. Size of red corpuscles ; 3200 of an inch. Size of white corpuscles ; 2507 of an inch. Shape of red corpuscles ; circular biconcave discs. Shape of white corpuscles; globular. Number of red corpuscles in a cubic millimeter of blood (the cubic zs of

an inch); 5,000,000.

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