The rapidity of the blood stream in the vessels is increased by active exercise. The increased strength and frequency of the heart-beat assists in this, also the increased positive pressure in the aorta (and in the pulmonary artery), the extremely low pressure in the venae cavae (and in the pulmonary veins), and all the different momenta referred to above, which help in driving on the column of blood in the veins of the systemic circulation on their way to the right heart.

The rate of the blood stream, which, other things being equal, stands in inverse ratio to the breadth of the path, is well known to be greatest nearest the heart, smaller in the veins, and less still in those which are further from the heart, least of all in the capillaries, which form the widest part of the circulatory stream. The speed varies enormously. Chauveau found it to be five to six times greater during movement than during rest. In the carotid artery of the horse it was 520 mm. a second during systole, 150 mm. during diastole. Tschuevsky found it up to 385 mm. a second in the carotid artery of a dog, Vierordt found it 0.8 mm. a second in the capillaries of the same animal - both of which results do not coincide badly with the usual theory that the stream in the capillaries is 400 times as wide as that of the aorta, and that, as already stated, the speed is in inverse ratio to the width of the stream.

Physical exercise has a definite and recognised influence on the composition of blood in diminishing the amount of water it contains, partly by increasing the amount of urine and perspiration given off, and partly by causing the protoplasm of the working muscles to become richer in water, so that the specific gravity of the blood is increased up to .006; the concentration of corpuscles is thereby temporarily increased. Of the absolute increase of the red and white blood corpuscles and of their increased use and destruction, both of which probably take place as a result of physical exercise, we know very little. That a sedentary life causes anaemia is an opinion often expressed by doctors and certainly correct. We have also reason to suppose that when the arteries of the muscles dilate as the muscle contracts the branches of these arteries going to the marrow of the long bones also dilate, and that the formation of blood in the bone marrow thus becomes more active. It is also reasonable to suppose that the activity of the other blood-forming organs, especially of the spleen and lymphatic glands, is stimulated because the circulation as a whole becomes more active. Contrasting with this favourable effect upon the formation of blood of muscular activity within physiological limits, is the severe anaemia known to follow extreme physical strain, especially the attacks of severe general spasm in tetanus (the infective disease).

By its strengthening effect upon the heart-beat physical exercise helps to increase the blood-flow through the tissues, and thereby the rate at which oxygen is supplied and C02 removed. Tissue respiration is thus improved.

As we understand from the above, physical exercise increases the work of the heart.

We assume now that each of the ventricles at each systole presses out 60 to 70 cubic cm. of blood (into the aorta and pulmonary artery respectively). If the latter figure is taken, and the pressure in the aorta is taken as represented by a column of blood 1.5 m. high, the product of these results in work amounts to 0.08 kgm. for each systole of the left ventricle. If the analogous figure for the right heart is put at 0.03 kgm., we obtain, with O. Langendorff, a daily amount of work for the heart of something over 11,000 kgm. in twenty-four hours. Other modern authors have arrived at 12,400 kgm.

In these estimates the inconsiderable amount of work of the auricular appendages and auricles has not been considered. During physical strain the work of the heart can be very considerably increased, but certainly not nearly to the extent supposed by the older authors.

I would point out in this connection a fact which is not sufficiently recognised in the medical world, that, owing to the influence of physical exercise on the heart's action, one can indirectly exercise a weak heart as one directly exercises weak skeletal muscles. This is the case to a remarkable extent in Stokes' (often inaccurately called Oertel's) "terrain-cure." In similar "cures," and in their influence upon respiration and ventilation of the lungs, we have an excellent therapeutic means of treating certain diseases and morbid conditions of the lungs (see later in the special part of the book).

* * * * *

The influence of active movements upon the circulation includes also effects on inflammatory processes and the products of inflammation.

Since active movements replete the part, they may also increase inflammatory processes. Neither is there any doubt that excess of muscle action may, on account of the strong hyperaemia to the working muscles, produce acute muscle inflammation; similarly, inflammation of tendon-sheaths is accounted for, both by patients and doctors, by muscular over-strain.

In acute inflammation local or general rest is the first indication.

On the other hand, we cannot doubt that hyperaemia promotes reabsorption, and, with products of inflammation other than pus, especially in chronic infiltrations due to myositis, movements act beneficially, partly by hyperaemia of the muscles, partly also by the alternations of pressure arising in the muscular tissues.

As a means of promoting reabsorption, however, medical gymnastics is far inferior to massage.

*****

Muscles by their activity exert a marked influence both on the intake of oxygen and on combustion (oxidation) within the organism.

Pulmonary ("external") respiration, or that process by means of which C02 from the blood is given off in the lungs and oxygen taken in from the atmospheric air, takes place largely through muscle action. The muscles of respiration, with which in this paragraph I am particularly concerned, form a special part of the motor apparatus, though other muscles which are equally part of this apparatus also sometimes assist in respiration.