We now assume on good grounds that there is a direct stimulation of the respiratory as of the cardiac nerve centres, caused by the above-mentioned substances, and have discarded the former explanation of reflex stimulation from the tissues thirsting for oxygen, and from local dyspnoea.

It has already been shown that deeper and more frequent respirations in this way accelerate pulmonary circulation, and that the pulmonary vessels are lengthened and receive more blood during inspiration, but during expiration become shorter and contain less blood.

I must also mention in this connection the fact, so difficult for those with little knowledge of physiology to understand, that deep inspirations strongly affect the circulation, because the lungs, owing to their elasticity, try to contract more strongly the more they are enlarged by inspiration, and, by means of this stronger effort, further lower the. pressure in the thorax, which was already negative.

As every student of physiology knows, the lungs are developed in the thorax in such a manner that their elasticity is always taken into consideration; in other words, if the thorax is opened from outside, so that the atmospheric pressure is as great on the external pleural surface of the lungs as that on the inside through the air passages, the lungs will contract. Owing to this elastic attempt to contract continually counteracted by the atmospheric pressure through the air passages, there exists in the thorax outside the lungs - except during very quick expiration and strain - always a negative pressure, which in normal position of expiration only reaches a few ( - 5 or - 6) millimetres mercury, but during a strong inspiration, when the lungs, being expanded to the utmost, attempt to contract with their maximum force, sinks to even - 30 mm. of mercury (below atmospheric pressure).*

Deep inspirations favour the diastolic movements of the heart by the strong negative pressure outside it in the thorax, and deep expirations assist the systolic movements, though indirectly, by continually diminishing this negative pressure. During deeper respirations the heart therefore works more strongly - alternately as a suction and as a pressure pump.

By their influence on respiration active movements work also upon the circulation by causing the movements of the diaphragm to become stronger, deeper, and more frequent. When the dome of the diaphragm from being arched up into the thorax is lowered by its contraction, the pressure in the abdominal cavity is raised and the blood is more strongly forced from the abdominal vessels than during rest, but probably only with moderate force (Hasse, Walz).

K. Walz has called this influence of forced inspiration upon the liver "liver-massage." He found (1901) that circulation in the liver and the emptying of the gall-bladder was promoted in this way.

Blood pressure, which depends on many circumstances (see p. 58), but to a marked degree on the strength of the heart's action and on the resistance in the arterioles, is affected in many different ways by physical exercise. By the dilatation of the vessels in the muscles during work pressure naturally sinks in these vessels and centrally from them, and this lowering has been beautifully demonstrated in horses and may reach 25 mm. mercury (Chauveau and Kauffmann). But this lowering is compensated by many things, among others certainly by reflexes from the vaso-con-strictors. The heart's stronger action increases pressure, and especially is it true that blood pressure, perhaps after a short fall, undergoes a decided rise during muscular work, which in man may attain even to 100 mm. Hg. in the larger arteries. This is a marked increase, if one takes the whole normal pressure to be from 120 to 150 mm., and quite sufficient to form a decided contra-indication to all strong movements in cases where there is danger of rupture of blood vessels.

* This influence which the lungs exercise upon the pressure in the thorax, on the structures inside it though outside the lungs, is the pons asinorum for many medical gymnasts. They cannot understand that the expansion of the thorax by muscular contraction in inspiration is the primary act, and that the expansion of the lungs when there is no pressure outside and atmospheric pressure inside is the result of the expansion of the thorax. It is therefore often difficult to change their fundamental idea that during inspiration the enlarged lungs press upon the other organs in the thorax. (N.B. - Negative pressure = a pressure that is less than atmospheric pressure.)

With Riva-Rocci's (Recklingshauscn's) apparatus the normal blood pressure in the brachial artery is reckoned at 150 mm. (In severe arteriosclerosis it may reach even 300 mm.) Even in the capillaries we have normally 25 to 30 mm., in the veins of the head 7.5 to 12 mm. In the venae cava? in the thorax the normal is always at least several millimetres negative pressure. In the pulmonary circulation the same rule naturally applies as in the systemic circulation, and similar conditions, but the figures and alternations are less. In the pulmonary artery of a dog the pressure is about 20 mm.

The greatly increased blood pressure caused by strain serves as a warning to medical gymnasts. Strain is brought about by closing the glottis with the thorax in the position of inspiration, and innervating the muscles of the abdominal wall, especially rectus abdominis, the oblique muscles, and transversalis. It is a common occurrence, especially during defaecation, and is easily produced in many different muscular actions. It is not, however, normal, and must be avoided in medical gymnastics; it is very dangerous in all conditions where there is danger of haemorrhage. As we all know, it has a hampering effect upon the circulation, in that it changes the negative pressure outside the lungs in the thorax to positive, so that the return of blood to the right heart by the venae cavae is made difficult. Stasis arises in the systemic circulation, and in severe strain makes itself obvious by the altered colour of the face; at the same time the pressure in the whole systemic circulation is considerably raised.