CH. XIV. DOUBLE CIRCULATION OF THE BLOOD. 113

returns by the lower vein, while the blood of the upper artery is returning by the upper vein, and both streams pour into the right upper chamber of

the heart, b.

The blood has now made one round, but it does not stop here. It escapes through some valves down into the lower chamber c; out of the right top corner of which it starts again in the direction of arrows 8 and 9, and passes

UPPER VEIN

RICHT

LUNG ARTERY

LOWER VEIN

HEART

ARTERY

LEFT

LUNG VEIN

LOWER ARTERY

Diagram of Heart and Blood-vessels

front.

through the lungs, returning by the lung-veins, or pulmonary veins as they are called, in the direction of arrow 10, back into the left top chamber of the heart, d. From there it passes down into the chamber a, from which it first started, and the whole round begins again. The first journey of the blood round the whole body is culation, and the second journey through the lungs is called the pulmonary circulation; when Harvey had traced these two journeys he had proved the double circulation of the blood.

ac, Lower chambers of the heart, called ventricles. bd, Upper chambers of the heart, called auricles. The arrows and numbers show the course of the blood.

called the general cir

Although this discovery as stated here appears very simple, yet it took Harvey nineteen years to trace the blood through all the channels of the body, before he felt quite certain that he had hit upon the truth. Meanwhile he had returned to London, and had been made physician at St. Bartholomew's Hospital. Here he taught his theory in his Lectures of 1619, and at last published a small book on the

circulation of the blood in 1628. Yet none of the older physicians would believe he was right, and Harvey told a friend that he lost many patients in consequence of his new doctrine. It is greatly to the credit of the unfortunate King Charles I., who was reigning at this time, and whose private physician Harvey was, that he gave him many opportunities of making physiological experiments on the animals in the royal parks, and took great interest in his discoveries. Harvey wrote several other valuable books, and traced the development of the chicken in the egg. He was of a very gentle and modest disposition, and disliked controversy so much that he could scarcely be persuaded to publish his later investigations when he found what disputes were occasioned by his great discovery of the circulation of the blood. He died in 1657, in his eightieth year.

Discovery of the Vessels which carry Nourishment to the Blood, 1622-1649.-Harvey's doctrine of the circulation. of the blood was the real starting-point of physiology, or the science of living bodies, and when the true action of the arteries and veins was known, many other vessels of the body were soon better understood. The most important of these were the vessels which carry nourishment from all parts of the body to make fresh blood. In 1622 Gaspard Asellius, Professor of Anatomy at Pavia, saw a white fluid flowing from some thread-like tubes in the body of a dog which he was dissecting. This dog had been eating food just before he died, and Asellius found that the fluid came from the intestines and was the nourishing matter of the food. He called these fine tubes lacteals, because the fluid in them looked like milk. Some years later, in 1647, Jean Pecquet, an anatomist of Dieppe, discovered that these lacteals empty themselves into a large tube called the thoracic duct, which

CH. XIV. FURTHER PHYSIOLOGICAL DISCOVERIES. 115

carries the fluid into the principal vein, and so to the heart; and finally, in 1649, a Swede named Olaüs Rüdbeck discovered an immense number of fine thread-like tubes running from all the principal parts of the body, and carrying nourishing matter to the thoracic duct, and so through the great vein to the heart. He called these tubes lymphatics; but in reality the lymphatics and lacteals are the same vessels, coming from different parts of the body and supplying the material for new blood. You will easily understand that when physiologists knew not only how the blood circulates through the body, but also how a fresh supply of blood is being constantly provided, they had made a great step owards tracing out the working of a living body.

Chief Works consulted.—Sprengel, 'Hist. de la Médecine,' 1815; Harvey's Anatomical Exercises,' 1673; Aikin's Biog. Mem. of Medicine till the Time of Harvey,' 1780; Huxley's 'Elementary Physiology;' Carpenter's 'Physiology;' Kirke's Physiology;' Cuvier, 'Hist. des Sciences, &c. ;' D'Orbigny, 'Dict. des Sciences.'

CHAPTER XV.

SCIENCE OF THE SEVENTEENTH CENTURY (CONTINued). Torricelli discovers the reason of Water rising in a Pump-Uses Mercury to measure the Weight of the Atmosphere-Makes the First Barometer-M. Perrier, at Pascal's suggestion, demonstrates variations in the pressure of the atmosphere-Otto Guericke invents the Air-pump - Working of the Air-pump-Guericke proves the Pressure of the Atmosphere by the experiment of the Magdeburg Spheres-He makes the first Electrical Machine-Foundation of Royal Society of London and other Academies of Science.

Torricelli's Invention of the Barometer, 1644.-We must now turn to quite another subject on which new light was being thrown at this time. Among the many different mechanical experiments which Galileo made during his life, there had been one with a common pump which puzzled him very much, and which he had never been able to explain.

You know that if you put the mouth of a squirt in water and pull back the handle, the water rises up into the tube. That is to say, as soon as you leave a space inside the squirt quite empty without any air in it, the water rushes in. In the same way, water may be made to rise up a long tube standing with its open end in a pond or basin, by drawing up a tight-fitting stopper A, Fig. 14, called a piston, and so driving the air out at the top and leaving a vacuum inside the tube. But Galileo found that as soon as the water had risen up to the height of about

CH. XV.

TORRICELLI-THE BAROMETER.

117

34 feet it would not mount any higher, even though the tube between the surface of the water c, and the piston a, had no

air in it. He could not, however, find out why the water should stop rising just at this point, and it was not till after his death that his friend and follower Torricelli (born 1608), who was a mathematical professor at Florence, hit upon the reason.

Torricelli asked himself, 'Why does the water rise in the tube at all? something must force it up.' Then it occurred to him that air must weigh something, and that it might be this weight on the open surface of the water which forced the water up the where there was no pump air pressing it down. To understand this you must picture

W

FIG. 14.

Ab

C 34 feet

Section of a Suction-tube.

W

to yourself all the air round our globe to be pressing down upon the surface of the earth. Now, so long as the tube

also is full of air the surface of the water will all be equally pressed down, and so will remain at one level at w B W. But when the piston a is drawn up, it pushes the air above it out of the tube, and so lifts the weight off the water at B, which will immediately be forced up the tube by the pressure of the air on the water outside from w to w. This will go on till the water has risen about 34 feet to c and