A further conservative provision is found in the manner in which the bloodvessels penetrate the brain. The larger arterial branches run in sulci between convolutions, or at the base of the brain ; smaller branches come off from them, and ramify on the pia mater, breaking up into extremely fine terminal arteries, which penetrate the brain ; or these latter vessels spring directly from the larger branches, and enter the cerebral substance. As a general rule, no vessel penetrates the cortical layer of the brain, which, in point of size, is more than two removes from the capillaries ; and, when-: ever any vessel of greater size does pierce the cerebral substance, it is at a place where the fibrous matter is external, and that part is per forated by foramina for the transmission of the vessels. Such places are the locus perforatus, the interpeduncular space, &c. The capillaries of the- cerebral substance are easily seen to possess an independent diaphanous wall, with cell-nuclei disposed at intervals. The smaller arteries and veins can also be admirably studied in the pia mater of the brain.
The venous blood is collected into small veins, which are formed in the pia mater at various parts of the surface, and in the interior of the brain. The superficial veins open by short trunks into veins of the dura mater, or into the neighbouring sinuses; the superior longitudinal, the lateral, and the straight sinuses receiving the greatest number. Those-from the interior form two trunks, yen& map:cc Galeni, vvhich pass out from the ventricles between the layers of the velum interpositum. The cere bral veins are devoid of valves.
We remark here, that the venous blood of the brain is returned to the centre of the circu lation through the same channel as that of the dura mater, of the cranial bones, and of the eyeball : the intemal jugular veins are the channel towards which the venous blood of the cranium tends. An obstacle, therefore, in both or either of these vessels must affect the entire venous system of the brain, or at least that of the corresponding hemisphere. A ligature tied tightly round the neck impedes the circulation, and rnay cause congestion of the brain. The bodies of criminals who have died by hanging exhibit great venous congestion, both of the walls and the contents of the cranium, in con sequence of the strong compression to which the veins have been submitted.
We have seen that, when the blood of one carotid artery is cut off, the parts usually sup plied by it are apt to become exsangueous and softened ; and this is more especially the case if the vertebral be stopped up, or the circula tion in it irnpeded. And it has been remarked,
that these effects will follow the application of a ligature to either common camtid artery.
Notwithstanding these facts, a doctrine has received very general assent, and the support of men of high reputation, which affirms that the absolute quantity of blood in the brain cannot vary, because that organ is incom pressible, and is enclosed in a spheroidal case of bone, by which it is completely exempted from the pressure of the atmosphere.
The cranium, however, although spheroidal, is not a perfectly solid case, but is perforated by very numerous foramina, both external and internal, by which large venous canals in the d iploe of the bones communicate with the cir culation of the integuments of the head as well as with that of the brain ; so that the one can not be materially affected without the other suffering likewise. And as the circulation in the integuments is not removed from atmo spheric pressure, neither can that which is so closely connected and continuous with it, be said to be free from the same influence. Still it must be admitted, that the deep position of the central vessels, and the complicated series of channels through which they communicate with the superficial ones, protect them in some degree from the pressure of the air, and render them less amenable to its influence than the vascular system of the surface.
If it were essential to the integrity of the brain that the fluid in its bloodressels should be protected from atmospheric pressure (as the advocates of this doctrine would have us to believe), a breach in the cranial wall would necessarily lead to the most injurious conse quences ; yet, how frequently has the surgeon removed a large piece of the cranium by the trephine without any untoward result l Some years ago I watched for several weeks a case in which nearly the whole of the upper part of the craninm had been removed by a process of necrosis, exposing a very large surface to the immediate pressure of the atmosphere ; yet in this case no disturbance of the cerebral circula tion existed. In the large and open fonta nelles of infants we have a state analogous to that which art or disease produces in the adult : yet the vast majority of infants are free from cerebral disease for the vvhole period during which their crania remain incomplete ; and in infinitely the greatest number of cases in which children suffer from cerebral disease, the pri mary source of irritation is in some distant organ, and not in the brain itself.