tids, and the two smaller vertebrals. It is returned by two veins: the internal jugulars. It has been stated that the arteries, on entering the skull, make a tortuous and indirect entrance, so as to diminish the impetus of the blood; and that this is more particularly the case in some of the lower animals, as the horse. In man, from the usual position of the body when awake and in a state of activity, the blood has to be propelled upwards, contrary to the influence of gravity, and therefore the serpentine direction to an equal extent is not required in him. The effect of position upon the circulation through the head must be well known to every one who has laboured under headache, who will recollect how much the pain is aggravated by stooping. This may, however, partly arise from obstruction to the return by the veins, as well as from the increased flow by the arteries.

In entering the skull, the carotids pass close to the internal ear, so that on increased sensibility of that organ, or inordinate excitement of the arterial pulsation, their throbbing can be distinctly heard. So soon as the four arteries have entered, the two vertebrals are formed into one, as has been mentioned; besides, there is a free communication established with each other by all the four. They then begin to divide into branches, which do not however enter immediately into the delicate nervous substance of the brain, but are first minutely subdivided upon a membrane which closely embraces its surface, and enters between its folds. So minutely are these vessels ramified before they penetrate into the nervous matter, that few are capable of transmitting the red globules of the blood; though the larger branches freely communicate before they subdivide, yet they appear to form few connections in their smaller branches. If any of the four trunks be obstructed, or if one of them be tied, as is sometimes the case, by the surgeon, in consequence of the connection among them, the blood is not cut off from the branches of that which is obstructed. There is another

circumstance which characterizes the arteries of the brain : their coats are much thinner and more delicate than in any other organ, for the strength and thickness of the coats of arteries vary very much in different parts; those of the lower extremity, for example, are much thicker and tougher than in most other parts; but here they are more tender than any where else. Hence they are more apt to give way, producing a fatal effusion of blood.

The brain in the living body, and after death, so long as it retains its vital warmth, is much softer than when it becomes cold. It undergoes a kind of coagulation after death, whereby it becomes firmer, until decomposition commences. In the living state it may be said to be semifluid, and hence probably the necessity for the delicacy of the coats of its blood vessels. The veins returning the blood are at first distributed also upon the membrane. They never accumulate into large branches, but instead of doing so, they pour their contents into receptacles for the purpose, termed sinuses, to which we shall immediately refer. In most parts of the body the larger veins accompany the larger arteries, but here that is not the case; for whereas the arteries divide from the base towards the vertex, the veins collect their branches from the base, and form larger at the vertex, where they enter the sinuses. Another circumstance influencing the circulation in the brain is the effect of the strong unyielding vault of the skull in which it is contained, whereby the absolute quantity of blood is at all times the same, the atmospheric pressure supporting it, even independently of the vascular action on the one hand, and the vault of the skull preventing the vicissitudes of atmospheric pressure on the other. It has been ascertained that if an animal be bled to death, there will yet remain as much blood in the skull as if it had been killed without shedding a drop of blood. But if a portion of the skull be sawn out, and an opening made

through the dense membrane which lines it, and the animal then bled to death, the vessels of the brain will in this case be found as much blanched as those of any other part of the body, in consequence of the atmospheric pressure having now had an opportunity of exerting its influence upon them.

Although, then, we may diminish the force and the velocity with which the blood circulates through the brain, we cannot diminish the quantity,-we mean in the whole cavity. That there may be a loss of balance is true: that is to say, it may be determined to, or congested in particular parts. Veins are liable to accumulation of blood from the nature of their coats. Such accumulations are, however, incompatible with the office and safety of the brain, and therefore, to prevent the risk of such an occurrence, the veins of the brain are not suffered to collect themselves into trunks, but pour their contents into the sinuses. These are placed in the folds of the dense unyielding membrane which lines the skull or dura mater, so that the venous blood in any considerable quantity can only accumulate in these receptacles, which, from their strength, resist distension, and have, even in their interior, transverse bands, named Willis' cords, which further act towards the same end. The manner in which the veins enter is also worthy of remark. The current of the blood in the sinus is in the direction of the arrows a a, that is, from before backwards, while in the veins it is in the direc

b

FIG. 8.

a

b

tion of the arrows, d d d, from behind forwards. The reason is this: It has been already mentioned that when the chest is enlarged, as in inspiration, there is a freer passage for the blood into it, but that when it is contracted in expelling the air, there is an impediment to its entrance; that, therefore, the veins are scen occa

sionally to swell. From the right auricle, along the descending cava and internal jugular vein, up to the sinus, there are frequently no valves, so that any impediment to the entrance of the blood into the chest or auricle causes a rise, while its free entrance produces a subsidence throughout the whole extent, or a flux and reflux, which is sometimes called venous pulsation. In laborious breathing, especially, this takes place. Now, if the reflux extended to the veins of the brain, it might, and most probably would, prove fatal; but as the reflux in the sinus will be in the direction of the arrows bb, coincident with the current in the veins, and of the arrows d d d, the obliquity of the entrance will therefore prevent the blood from re-entering the veins, though there will be an obstruction to their emptying themselves into the sinus, which explains what occurs under such circumstances as the following. When the brain is compressed, as from a fracture of the skull, and the depression of the fractured portion, the breathing becomes affected, it takes place at intervals, or is rendered stertorous. In the performance of the surgical operation required in such cases, the brain is seen to rise and fall-to rise during expiration, and subside during inspiration: that is, during expiration there is an impediment to the entrance of the blood into the sinus from the veins; it consequently accumulates in them, and the brain swells, but in inspiration the current of the blood becomes free, and the brain subsides. Thus we see in the transmission, in the distribution, in the collection and return of the blood to and from that noble and delicate organ the brain, everything is foreseen and provided for, nothing left to chance, overlooked, or omitted.

We have next to turn our attention to the peculiarities to be found in the circulation of the blood through the abdomen. That cavity is chiefly occupied by the organs which are either directly or indirectly engaged in the preparation of the food for its entrance into the living

vessels, and for the introduction of fluids to compensate for the loss every moment sustained by the general circulating mass.

The motion to which the viscera of the abdomen are constantly exposed, in consequence of the respiratory movements, undoubtedly has an effect in promoting the circulation through them. The stomach and intestinal canal have a muscular coat which produces a peculiar motion hereafter to be described. This is altogether state of health we

independent of the will, and in a are unconscious of its existence. This motion is slow and uniform, varying in these respects according to the habit of the individual, and other circumstances. The circulation through the stomach, intestinal tube, spleen, pancreas, and certain folds of the lining membrane of the abdomen, is different from that of any other part of the body. The blood is, as elsewhere, transmitted by arteries; as elsewhere, it is collected by veins; but instead of directly conveying it to the heart, these collect themselves into one trunk, which proceeds to the liver; there it again divides through the substance of that organ, after the manner of an artery, that is, distributing the blood from trunks to branches. Again it is taken

FIG. 9.

or more

up by veins, which are collected into one trunks; and these pour into it the ascending cava to be