of excitation of the vagus centre, the heart beats more slowly or is arrested. In consequence of excitation of the vasomotor centre, the small arteries throughout the body are contracted and the arterial blood-pressure is raised. Excitation of the bulb producing these effects may be caused in several different ways-1, by direct electrical stimulation of the bulb itself; 2, by stimulation of the bulb by venous blood; 3, by reflex action of the bulb in consequence of stimulation of afferent nerves; moreover, the vasomotor centre may vary in its actions spontaneously-i.e. without assignable cause; and in the moribund state when respiration has already ceased, it may rhythmically wax and wane in action, before it ceases to act altogether (p. 149).

From what has been said it will be clear that of the two possible vasomotor changes-constriction and dilatation-the former is the dominant and more marked change; if all the

Excitation of the central end of the depressor nerve. Fall of blood-pressure due to reflex relaxation of the splanchnic vessels. (N.B. The vagus was uncut, hence the fall is associated with reflex slowing of the heart-beat. The abscissa has been raised 3 cm.)

vasomotor nerves of the body are stimulated, as by stimulation of the spinal bulb, the resultant is vaso-constriction and rise of blood-pressure; conversely, if all the vasomotor nerves are put out of action, as by destruction of the bulb, the resultant is abolished vaso-constriction and fall of blood-pressure. As regards reflex vasomotor changes, the effects differ with the afferent nerve excited, and with the state of the vessels. The usual result of stimulation of an afferent nerve is a reflex rise of bloodpressure, but in the case of one particular nerve which is an afferent channel from the heart to the bulb-i.e. the depressorstimulation of the central end of the divided nerve gives a reflex fall of blood-pressure; the mechanism of this fall is peculiar-it is not produced if the splanchnics have been previously divided, the effect of their division being of itself a considerable fall of blood-pressure by dilatation of the intestinal vessels. It appears,

therefore, that the depressor is not in constant action and that it cannot always be brought into action by stimulation, but that a high blood-pressure is a necessary condition. And it is probable that normally it is brought into action when blood-pressure is so high as to embarrass the heart's action, the depressor then conveying from the heart impulses which depress the vasomotor centre, especially as regards its constant constrictor effect upon the intestinal vessels by way of the splanchnic nerves; these vessels being thereby relaxed, pressure falls, and the heart is relieved. This, indeed, is not the only way in which a high blood-pressure brings about its own remedy; high pressure gives rise to a slower heart-beat (by vagus-control), and with that slower beat, although the output at each beat is greater, the total output in a given time is less. We may now fully appreciate how the cardiac and arterial factors of blood-pressure are coordinated through nervous channels, effecting the physiological combinations alluded to above, viz. greater action of heart linked with vaso-dilatation, and smaller action of heart with vaso-constriction. A greater pulse-frequency has as its mechanical effect a greater blood-pressure; a greater blood-pressure has, as its physiological consequences, a reduction of blood-pressure (via depressor and splanchnic) and a reduction of pulse-frequency (via the vagus nerve). Guided by these considerations, we may realise that: a raised blood-pressure with a greater pulse-frequency, or a lowered blood-pressure with a smaller pulse-frequency, must be of cardiac origin; while a raised blood-pressure with a smaller pulse-frequency, or a lowered blood-pressure with a greater pulsefrequency, must be of arterial origin. The first two of these four cases are illustrated by vagus action, the last two by the effect of dyspnoea and of amyl nitrite.

Stimulation of the central end of any other afferent nerve usually gives reflex rise of pressure, but not always. Repeated stimulation produces each time less and less rise, until, finally, it may produce a fall. It would appear that vaso-constrictor action, by reflex as well as by direct experimental stimulation, wears out more rapidly than vaso-dilatator action, which being thus unmasked becomes evident. In chloral poisoning it is usual for stimulation of afferent nerves to give at once a fall, not a rise, of blood-pressure. It would appear that constricting action is more affected by chloral than dilating action. These results refer to the reflex effects of stimulation of afferent nerves; very similar results are obtained by the direct stimulation of

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efferent (vasomotor) nerves. The sciatic, for instance, if its peripheral end be stimulated, gives, if fresh, vaso-constrictionbut if fatigued, vaso-dilatation. Here again it appears that dilating outlasts constricting action. The effects are such as to remind us of those obtained by prolonged stimulation of the cardiac nerves; when vagus and accelerans are simultaneously excited, the vagus effect wears away comparatively quickly, and the accelerator effect is left unmasked.

Local vasomotor reflexes.-As has been stated, the effect of the stimulation of efferent nerves is usually vaso-constriction and rise of general blood-pressure, but sometimes the reverse. The local effects are precisely opposite; usually the circulation becomes more active in a part of which an afferent nerve is excited, the vessels of the part dilate, while the rise of the general blood-pressure shows that other vascular districts of the body at the same time contract; both factors thus concur in promoting the more copious blood-supply of the district, an afferent nerve of which is stimulated; the effect is produced by experimental stimulation of the central end of a divided nerve containing afferent fibres, or by irritation of the cutaneous periphery, the nerves of which are intact, or by excited action of a part. Irritation of the central end of the great auricular nerve (in the rabbit) and consequent dilatation of the vessels of the ear, irritation of the central end of the tibial nerve and consequent dilatation of the external saphena vein, are the two best-known instances of reflex vaso-dilatation by experimental stimuli; the effect of a blister is a familiar instance of reflex vaso-dilatation by cutaneous stimulation; the more active circulation in contracting than in resting muscle, in secreting than in resting gland, are physiological instances of reflex vaso-dilatation in consequence of increased activity of a part, to which may be added as a pathological instance the increased activity of the circulation in an 'inflamed' part. In all these cases peripheral activity causes centripetal stimulation, the reflex effects of which are vaso-dilatation and increased blood-supply.

It should finally be said that the contractility of blood-vessels -arteries as well as veins-may manifest itself independently of influences through nerves from the central nervous system. As regards veins, the most striking instance is that afforded by the bat's wing (Wharton Jones), in which rhythmic constrictions of the veins may be observed, occurring once or twice per minute, and-owing to the presence of valves-promoting the onward

course of the blood. As regards arteries, the fact that they can gradually recover tone' subsequently to nerve-section shows that tone is not entirely of central origin.

THE LYMPHATIC CIRCULATION

The lymph moves in a circle, but very slowly in comparison with the blood; it exudes into the tissue-spaces through the walls of the capillaries, and is carried back to the blood by lymphatic capillaries and vessels, converging from all parts to form the thoracic ducts, which discharge their contents into the subclavian veins. The forces by which the current of lymph is kept up are:-(1) the exudation pressure under which lymph is discharged, and (2) the accidental compression of lymphatic spaces and vessels by muscular movements, and by the arterial pulse throughout the body; the lymphatic vessels are abundantly beset with valves which permit fluid to pass towards the heart, but prevent it from being driven backwards towards the tissues; moreover, the walls of the larger vessels are contractile; a slight favouring action is attributed to the current of blood in the subclavian veins, and the aspiratory action of inspiration must act in the same sense; in the lacteals, in which valves are abundant, the flow of chyle is promoted by the pumping action of the villi and by any movement of the intestine.

With regard to the origin of lymph and the mechanism of its discharge, it is obvious that the blood is the ultimate source of all lymph; from the researches of the Leipzig school, under the guidance of Ludwig, we know that the two factors with which the lymph-flow rises and falls are-(1) blood-pressure, (2) muscular movements. But according to Heidenhain's researches, in addition to these mechanical factors, another variable force must be reckoned with, i.e. secretory activity-(1) of the epithelium forming the capillary walls, and (2) of the tissue-cells themselves. Heidenhain has shown further that the lymph secreted directly from the capillaries or indirectly from the tissues can be distinguished and separately influenced by the injection of various substances; e.g.,extract of crab's muscle favours the secretion of 'blood-lymph,' while urea, or salt, or sugar, favours the discharge of 'tissuelymph.' One of his most conclusive proofs of lymph secretion is afforded by the fact that the flow of lymph in the thoracic duct will continue for a considerable period in spite of occlusion of the

aorta and fall of blood-pressure, and that it is increased, in spite of reduced blood-pressure, after occlusion of the vena cava, or of the vena porta.

In some of the lower animals special contractile organs, the so-called lymph-hearts, by their rhythmic action, forward the movement of the lymph. The frog possesses two such pairs of hearts, an anterior pair beneath the scapula, and a posterior pair in the ileo-coccygeal space; their functional relations are in many respects analogous with those of the blood-heart; their nervous supply is derived from the second and from the tenth pairs of spinal nerves, and from the sympathetic; the muscle which enters into their composition is similar to that of the heart; and although the experimental effects of nerve-section and nerve-stimulation are not so precise as might be desired, we have evidence that the action of the lymph-hearts, while locally independent of the spinal cord, is subject to reflex inhibition like that of the heart itself. The rate of beat, usually ranging from 60 to 80 per minute, is however far more variable, and each lymph-heart has its own rhythm independent of that of the others.

Observations on the pressure and flow of fluid in the larger lymph-vessels indicate a far more sluggish circulation than in the case of the blood; in the thoracic duct the pressure is between 1 and 2 cm. of water, and the current flows at a rate below cm. per second, and the total amount of fluid traversing the duct per diem is less than 5 litres. In its passage through the lymph-glands the fluid receives newly-formed leucocytes, and becomes more highly charged with proteid. The glands just alluded to are essentially masses of lymphoid tissue, bordering upon what is termed the lymph-channel, through which the fluid percolates from afferent to efferent lymphatics of the glands. This lymphoid tissue is the source of leucocytes, and plays the part of a physiological filter in relation to lymphatic absorption; this is illustrated by the effects of poisoned wounds, such as are frequently received in dissecting or post-mortem work; inflammation set up by septic matter extends along the lymphatics to the lymphatic glands, where it usually remains localised.

The total amount of lymph in the body is estimated at 25 to 30 per cent. of the body-weight, i.e. no less than three or four times the amount of blood, but the data upon which the estimate is formed are very imperfect, and the estimate is quoted merely to signify that the amount is very large.