ESCAPE OF BLOOD FROM CRANIAL CAVITY. 27 cerebral venous channels in general), it allows of the transit of blood in whichever direction, there happens to be the least obstruction. Under ordinary circumstances, however, blood traverses the ophthalmic vein, in a direction from within, outwards towards the orbit; thus forming another of Nature's provisions, and perhaps the most important of all, under circumstances of sudden and temporary congestion of the brain, for the egress or escape of blood from within the cavity of the cranium. The absence of valves in the whole of these venous tubes, is a circumstance which is doubtless connected with a wise intention. It enables the blood to pass in either direction, and consequently, greatly increases the freedom of venous circulation; a point of essential importance with an organ, whose functional capacity is so liable to interruption under so slight a disturbance of the balance of its circulating fluid. There is a tendency, which is not only prevalent amongst students, but even amongst others, to ascribe to the internal jugular veins a more important part than they really perform; or to attribute to them a greater share in the return of venous blood from the brain than they really take; and to regard in a less important light than they really deserve those accessory streams which escape in various points through the osseous walls of the skull. Not only do these accessory streams convey from the brain a considerable portion of its venous blood, under the normal, but also under the abnormal conditions of life; for, when there exists a temporary venous obstruction in the lungs or heart, they constitute the chief and almost only means of escape of venous blood from within the cranial cavity. If, for example, from a voluntary effort, or from some other cause, the process of respiration be for a short time arrested, we know, as a matter of observation, that the eyes start, and that the face becomes exceedingly red and turgid. The temporary cessation of the respiratory action having produced a stagnation of blood in the capillaries of the lungs, the right auricle becomes gorged, and the whole venous circulation obstructed. The first effects of this obstruction in the lungs and at the right side of the heart, being thrown on the larger vessels in most immediate relation with the right auricle, the circulation in the jugulars becomes early impeded, and the cerebral organ for a time is relieved of its venous blood almost entirely through the medium of those smaller veins (amongst which the ophthalmics hold an especially conspicuous position), escaping through various parts of the cranial parietes to the exterior of the head. These veins being placed at a distance from, and therefore in much less direct communication with, the heart than the jugulars, are less influenced by a temporary engorgement of the right auricle; and, admitting as they do, from the distensible nature of their thin coats, and from the laxity of the surrounding tissues in which they ramify, of considerable dilatation, they are capable, for a limited period, of responding to the increased function that is thrown upon them, and of giving exit to the blood from the interior of the cranium,—a circumstance that accounts for the well-known appearances observed under such conditions. Where this venous obstruction has existed until death has supervened, as in cases where life has been destroyed by suffocation, strangulation, or by hanging, appearances of the same description, but of a much exaggerated character, are produced; and the starting of the eyes, which forms so prominent, so hideous, and so striking CAUSE OF STARTING OF EYES DURING STRANGULATION. 29 a feature in these unfortunate subjects, arises from venous distension, in the way I have just pointed out, and not from spasmodic muscular action, as has been by some supposed. If we reflect for a moment, it appears nothing less than an absurdity in the extreme, to attribute this protrusion of the eyes to muscular spasm. There are six muscles that move the globe, and of these six, two only can possibly have the power of drawing it forwards. How is it possible, then, for these two-the two obliqui-to overcome the other four-the four recti-which, compared even individually, are of far superior strength; and which, during a state of spasmodic action, would be converted into powerful retractors. General spasm, therefore, of the muscles of the eye-ball, would, instead of protruding it, unquestionably retract it. But, it is not only from such an à priori consideration that we need arrive at such a conclusion, for actual observation can supply evidence of an equally conclusive nature. During that truly formidable and fearful disease, tetanus, the progressive symptoms may be watched from day to day, or from hour to hour, as the disease advances in its onward strides, and successively implicates nerve after nerve, of the cranial and spinal systems. Now, immediately that the moto-ocular nerves are brought into subjection to its baneful influence, the globes of the eye become powerfully retracted, so as to give to the orbit that shrunken appearance that forms so characteristic a feature in the expression of these unhappy patients. Besides the numerous outlets that are observed for facilitating the escape of blood from the interior to the exterior of the head, and which tend to obviate the injurious effects of a temporary obstruction to its return 30 CONGESTION COUNTERACTED BY CEREBRO-SPINAL FLUID. through the jugulars, there is another natural arrangement which I shall allude to here, as it forms so wise and beautiful an adaptation for counteracting the effects of pressure during a temporary state of cerebral congestion. What I refer to, is a compensating medium in the form of a fluid-the cerebro-spinal fluid—which, within certain limits, accommodating itself to the amount of blood contained within the cranium, maintains a constant equilibrium in the degree of pressure to which the cerebral mass is subjected. Any augmentation in the quantity of blood within the cranial cavity, would necessarily exert an undue pressure on the brain, were there not a corresponding displacement of cerebro-spinal fluid into the vertebral canal, to counterbalance by its decrease the increased space occupied by blood. This escape of cerebro-spinal fluid from the cavity of the cranium, into that of the vertebral canal, is permitted by an equivalent displacement of blood, from the immense spinal plexus of veins which surrounds the outer membranous investments of the cord, and which seems arranged in special reference to this particular object. The extent to which this compensating power of the cerebro-spinal fluid can be exercised or the limit of its capacity of displacement from the cranial cavity, is sometimes well illustrated in cases of apoplexy, where the blood has found its way into the ventricles of the brain. In such instances it occasionally happens that the blood coagulating after its effusion, forms a mould of the space it occupied, and thus gives a precise indication of the exact extent to which it has reached in its progress towards the vertebral canal. In this pathological specimen on the table before me, we have EXTENT OF DISPLACEMENT OF CEREBRO-SPINAL FLUID. 31 a preparation of such a description. You will observe that the blood having made its way into one of the lateral ventricles, passed from thence through the foramen of Munro into the third ventricle, and from the third ventricle along the iter a tertio ad quartum ventriculum into the fourth ventricle, where its progress was arrested by the incapacity of the cerebrospinal fluid to admit of further displacement. Up to this point, the expulsion of blood from the spinal plexus of veins had accommodated, or given space for the escape of cerebro-spinal fluid from the ventricles of the brain into the vertebral canal; but as soon as the limit of this compensating capacity was attained, the continued effusion of blood then began to exert an undue pressure on the cerebral mass, which at first disturbed, but afterwards completely arrested its functions, and with these, the functions of life also, immediately that the influence of the compression reached the medulla oblongata-the portion of the nervous centre, associated with the processes of respiration and deglutition. At present I have only assumed that, under the augmentation of the normal amount of blood within the cranium, an escape of cerebro-spinal fluid takes place into the vertebral canal, and that this is here accommodated by an equivalent displacement of blood from the spinal plexus of veins. But it is not difficult to show, by actual experiment, the truth of this statement; and I do not know that any more conclusive evidence can be required than is furnished by these two simple experiments, which I performed, now many years ago, and which I have been since accustomed, annually, to mention in my anatomical course of lectures. In the first experiment I opened the abdomen of a |