PASSAGE OF CAROTID THROUGH CAVERNOUS SINUS. 47

the cavernous sinus-a sinus situated on each side of the body of the sphenoid. That an artery should thus actually traverse or pass through a venous sinus, and that it should lie, as it were, bathed or immersed in a lake of venous blood, certainly forms a striking and peculiar arrangement, and one which we do not observe presented in any other part of the human body. Such a peculiar relation between the venous and arterial system, as I have just pointed out, is doubtless not without some wise and designable end, and therefore deserves a careful inquiry into the effects it is likely to produce.

In other parts of the body, especially in the extremities, there are, as I have already observed, certain accessory influences for accelerating the venous circulation, which do not exist in the interior of the cranium. For example, during the ordinary movements of the different parts of the frame, which are so constantly taking place, there is with each muscular contraction a compression of the adjacent venous tubes; and, as these vessels are furnished with valves, disposed in such a manner as to prevent the retrograde course of blood, it is obvious. that the successive muscular contractions, by the compression thus exerted, constantly tend to urge onwards the fluid contained within these venous trunks, and in this way to accelerate the venous current in its transit towards the right side of the heart.

In the extremities, also, each principal artery is usually accompanied by two veins, lying in close contact with it, and placed one on each side. Now, each arterial pulsation must necessarily communicate an impetus or momentum to the blood circulating in the accompanying veins, and as the blood can only travel in one direction, on account of the existing valvular arrange

48

EFFECT OF CAROTID IN TRAVERSING CAVERNOUS SINUS.

ment, the influence of the communicated impetus will be, like the influence of muscular action, to accelerate the progress of venous blood towards the right side of the heart.

After these considerations we are now prepared for the explanation of the relation observed between the carotid artery and cavernous sinus within the cranium. The object of such a relation seems to be to act as a compensation for the accessory forces that elsewhere exist, and to facilitate the circulation of blood in the cranial venous channels communicating with the cavernous sinus. The position of the carotid artery within the sinus, is analogous, in a functional point of view, to what I have stated concerning the arteries of the extremities and their venæ comites. But, it is even placed under a still more favourable condition than these for the production of a similar effect; for the walls of the artery being immersed or bathed in the pool or lake of blood constituting the sinus, each pulsation of the arterial tube communicates a considerable impulse to the surrounding fluid, which escapes from its enclosed cavity or sinus in the direction in which it meets with the least resistance, there being no valves in any of the venous channels situated in the interior of the cavity of the cranium.

The pulsations of the carotid arteries in their passage through the cavernous sinuses, thus supply the absence of that accessory influence which the venous circulation in other parts of the body receives from the muscular system. And, the momentum derived from such a continued series of impulses is fully sufficient not only to prevent the stagnation of blood in the cranial sinuses, but to urge it quickly onwards towards the right side of the

PROCESSES OF THE INTERNAL BASE.

49

heart, in the direction which offers the least obstruction to its progress. If there be congestion of the venous tubes of the interior of the cranium, it then passes, as I have before pointed out, by the ophthalmic veins through the orbits on to the exterior of the face; where, from the little resistance offered by the surrounding tissues to venous dilatation, it can find room within certain limits for its escape.

I have thus far opened the discussion of some of the relations presented by the arrangement of the internal aspect of the walls of the cranial cavity; and the remarks that I have even as yet made will, I think, suffice to show you that the processes of bone projecting from the internal surface of the cranial base, instead of being adapted, as would at first sight appear, for piercing, lacerating, and destroying the brain, carry out a wise and beneficent design in the cranial economy: namely, that of receiving vibrations from the cranial parietes and of transmitting them to the cerebro-spinal fluid in which they are immersed; and in which, therefore, such vibrations are interrupted or lost before reaching the cerebral tissue, thus obviating that disturbance of the cerebral functions, which would be otherwise constantly accruing from the most trivial blow or concussion of the head.

In continuing this subject, I shall proceed with a more detailed investigation of the special associations that suggest themselves in relation to the several processes, examined separately, of the internal base; and in doing this, let us commence with the crista galli of the ethmoid, and thence pass in a direction backwards towards the occipital bone.

The crista galli consists of a prominent plate of bone,

E

which is so situated as to prevent the anterior lobe of the brain of one side, from pressing on the olfactory nerve of the opposite, during the lateral inclinations of the head. The necessity of such a provision becomes apparent when we take into consideration the extremely soft and delicate nature of the olfactory nerve-a character it presents to a far greater extent than any other cranial or spinal nerve, and which would render the slightest amount of pressure injurious to its integrity.

Besides the evident protection thus afforded by the crista galli to the olfactory nerve, it is further secured by being lodged in a longitudinal groove, or sulcus, on the under aspect of the anterior lobe of the cerebrum ; which, I may observe, is the only longitudinal groove we meet with on the base of the brain.

That the protective influence here referred to, however, does not form the whole intention of the crista galli, is rendered obvious by the great strength and thickness it presents. For, if its function were merely to protect the olfactory nerves from pressure, a thin plate of bone would fully answer all that was required. Its strength and thickness, therefore, may be presumed to be developed in relation to some other intention; and, on searching for an explanation, we find that it forms a fixed point for the attachment of the falx cerebri-an exceedingly strong process of the dura mater, that separates the hemispheres of the brain from each other; and also, that it constitutes one of those projecting points from the internal base, that I have already alluded to, as intended for the salutary termination of cranial vibrations. Vibrations, indeed, converging from a portion of the cranial parietes, are conducted to the crista galli, where they become obstructed and lost in the membra

ANTERIOR CLINOID PROCESSES.

51

nous texture of the dura mater, to which it is so largely and closely attached.

This connexion of the crista galli with the termination of cranial vibrations, receives corroboration from the fact, that a tolerably severe blow on the back of the head not unfrequently leads to an attack of hæmorrhage of the nose. Now, the hæmorrhage in such cases, arises, I believe, from the conduction of vibrations along the ridges of the groove for the longitudinal sinus to the crista galli, and from thence to the septum of the nose, with which this process is in direct continuity: some of the small viens in this neighbourhood being thereby ruptured will fully account for the phenomenon that is observed.

The anterior clinoid processes form strong points of bone, that project backwards into the interior of the cranial cavity from the alæ minores of the sphenoid. The first idea that suggests itself, with reference to their functional associations, is derived from the anatomical relation they bear to the internal carotid arteries. These vessels, after emerging from the petrous portions of the temporals, ascend in a direction upwards and forwards along a shallow groove that may be noticed on each side of the body of the sphenoid. They then pass towards the cerebral substance, and in doing so are always situated on the inner side of the anterior clinoid processes; which, thus counteracting their tendency to separation during each pulsation, consequently prevent that laceration of their communicating transverse branch, which might otherwise, in all probability, ensue. I have never seen or heard of the internal carotids lying external to the anterior clinoid processes; and sometimes the design just referred to is still more perfectly carried out by the