has been destroyed (pithed) while the circulation is being studied. The small arteries dilate and the pulse becomes apparent in the capillaries, and even in the veins.

It seems probable that in the medulla oblongata a vasomotor centre exists, which can regulate the contraction of all the vessels, and keep them constantly more or less contracted. This slight general vascular constriction is spoken of as the arterial tone. The existence of such a centre in the medulla, and of nerve channels in the cord leading from it, is made certain by the fact

FIG. 143.

T

мишшшшшшшшшшш

Kymographic tracing showing the effect on the blood-pressure curve of stimulating the central end of the depressor nerve in the rabbit. The recording surface moving from left to right. (C) Commencement and (O) cessation of stimulation. There is considerable delay (latency) in both the production and cessation of the effect. (T) Marks the rate at which the recording surface moves, and the line below is the base line. (Foster.)

that if a gentle stimulus be applied to a certain part of the medulla, or just below it, simultaneous general vascular constriction sets in, as indicated by a great and sudden rise in the blood pressure.

Pressor Influences.-The action of the vasomotor centre can be increased, the tone of the vessels elevated, and the pressure raised, either by (1) direct or (2) reflex excitation. Directly, if the blood flowing through the medulla contains too little oxygen or too much waste products it stimulates the centre and the

blood pressure rises. This may be seen by temporarily suspending artificial respiration during an experiment on blood pressure, when the pressure rises considerably. Reflexly, the activity of the vasomotor centre can be increased by (1) the stimulation of any large sensory nerve or (2) by sudden emotion (fear).

Depressor Influences.—The tone of the arteries may be diminished by inhibiting the activity of the vasomotor centre by the stimulation of a certain afferent nerve, the anatomy of which has been made out in the rabbit and some other animals, and probably has its analogue in man. It passes from the inner surface of the heart to the vasomotor centre in the medulla. The effect of stimulation of this nerve in lowering the blood pressure is so great that it is called the depressor nerve. Some emotions (shame) may also reduce the activity of the centre, as seen in blushing, which is simply dilatation of the facial vessels.

Subsidiary Centres.-Besides this chief vasomotor centre it is probable that in the higher animals, as certainly is the case in the frog, other centres are distributed throughout the spinal cord which are able to take the place of the great primary centre. After the spinal cord has been cut high up, the hinder extremities more or less recover their vasomotor power in a few days, and destruction of the lower part of the spinal cord causes renewed vasomotor paralysis. In frogs this recovery takes place rapidly, the centres being less confined to the medulla than is the case in the more highly organized animals, but in the rabbit and dog it has been observed to occur more slowly.

Besides keeping up the normal tone, the arterial nervous mechanisms have the function of regulating the amount of blood supplied to various organs or parts at different times. Both vasomotor and dilator or inhibitory impulses are probably employed for this purpose.

REGULATION OF THE DISTRIBUTION OF THE BLOOD.

The various experimental results recently obtained on this subject (too numerous to be mentioned here), show that the vascular nerve mechanisms are very complex. The supposition of some such arrangements as the following may help the student.

1. The blood vessels have muscular elements which, though commonly controlled by nerves, are capable of automatic activity. A supply of arterial blood is sufficient stimulus for their moderate action, and mechanical or other local stimulus is capable of exciting increased constriction. We know that such automatic contractile elements exist in some of the lower animals (snail's heart, hydra, etc.), and we have no reason to doubt their existence in mammals. Moreover, such a view obviates the necessity of supposing that local nerve elements exist which cannot be recognized morphologically.

2. In the medulla oblongata there exist nerve cells which exert a constant influence over the activity of the vascular muscles. These groups of nerve cells which compose the vascular nerve centres may be divided into motor and inhibitory. From these centres impulses of two distinct kinds emanate, the one increasing the action of the contractile elements, and the other diminishing it. They are intimately connected with the centres which preside over the functional activity of the various viscera, and are also closely related to the nerves coming from all parts of the circulatory apparatus.

3. Direct communication between these vasomotor and vasoinhibitory centres and the blood vessels is kept up by means of efferent nerve channels, some bearing stimulating (vaso-constrictor) others inhibitory (vaso-dilator) impulses.

4. The activity of the contractile elements of any given vascular area may be altered by influences from different sources. (4) Local influences are brought but little into play, but, if the part be cut off from the nervous centres, they are capable of controlling the local blood supply by changing the degree of local arterial constriction. (3) Central influences from the medulla are habitually in action, affecting all the vessels and keeping up the vascular tone. These impulses are variously modified by changes occurring in distant parts of the circulatory apparatus, and can be regarded as a general regulating mechanism. They pass through the sympathetic chain. (7) Special influences, which are associated with the functions of the different parts and organs, are only called into operation during the performance of

the function, whatever it may be. These impulses are probably conveyed by the same nerves as excite the various forms of functional activity.

These three modes of regulation have different powers in different parts, and thus we find that section or stimulation of certain nerves gives vasomotor effects which appear contradictory.

Section of a sensory nerve causes temporary vasomotor paralysis, owing to the tonic constrictor influence being cut off. Stimulation of the peripheral stump causes vaso-constriction from excitation of the fibres bearing these impulses.

The stimulation of a motor nerve causes an increase in the flow of blood through the muscle, i. e., is associated with a vasodilator effect, probably dependent on the inhibitory influence of certain efferent fibres which check the local vascular agencies.

Thus we must suppose that there exist local agents under the control of the medullary centres, and that there are distinct sets of efferent, exciting and inhibitory fibres passing between the centre and periphery. One set of fibres lies in the ordinary functional nerve of the part, the other in the sympathetic, which to a great extent runs along the vessels themselves, and forms intricate networks.

As far as we know anatomically there are no local agents other than the muscles in the wall of the vessels. Since the impulses from the centres which can stimulate or inhibit the activity of the local agents travel by different fibres, all the observed phenomena may be explained without supposing local nerve centres to exist.

CHAPTER XVIII.

THE MECHANISM OF RESPIRATION.

The blood undergoes a series of modifications, and is constantly being altered as it passes from one part or organ to another.

It has already been seen that a quantity of nutrient material is taken up by the blood on its way through the capillaries of the alimentary tract, and a stream of lymph and chyle is poured into it when it reaches the great venous trunks; so that from two sources the blood is obviously increased in quantity. The most essential change that takes place in the circulatory fluid is the respiratory, and the addition it most urgently demands is that which it receives in the capillaries of the lungs. All the blood passes through these organs in order to ensure the elimination of the carbonic acid acquired in the general systemic capillaries, and the recharging of the red corpuscles with oxygen.

These gas interchanges will form the subject matter of the present chapter; and the more especial modifications which the blood undergoes in the ductless glands, the spleen, the liver, etc., as well as in the kidneys and other excretory glands, will be considered subsequently.

As has already been pointed out (Chapter V), an animal during its life may be said to use the substances supplied to it in food as fuel, and thus to acquire the energy which is bound up in them; for the activities of the various tissues are really combustions, being invariably associated with oxidation of some of the carbon compounds, so as to produce carbon dioxide and In order that the structures may be able to undergo this change they must have a ready supply of oxygen constantly at hand, and, moreover, the carbon dioxide which is formed in the process must be removed. The regular income of oxygen and the regular discharge of carbon dioxide are the first essentials to life; hence we find in almost all animals special arrangements known as the respiratory apparatus, by means of which these

water.