Occipital sinuses. —These are small veins lodged between the layers of the falx cerebelli. They collect the blood from the dura mater and from the cranial bones in the immediate vici nity of the posterior margin of the foramen magnum, and from thence they pass upwards and inwards to open into the lower part of the torcular. Cruveilhier suggests that the direc tion and position of the occipital sinuses are best indicated by describing them as being the cords of the arcs which the lateral sinuses form.
Petrosal sinuses. — These sinuses are so named from their connection with the petrous portion of the temporal bone. The superior petrosal sinus corresponds, on each side, to the posterior superior edge of the petrous bone, along the three outer fourths of which a groove exists for its reception. This groove is inter rupted in front by a depression in which the tifth nerve is lodged, so that at this place that nerve lies between the sinus and the bone. The superior petrosal sinus is about large enough to contain an ordinary sized surgeon's probe. It communicates with the lateml sinus posteriorly and with the cavernous sinus in front, and in its course it receives several small veins from the dura mater in the middle fossa of the cra nium. It receives a vein from the anterior por tion of the corresponding hemisphere of the cerebellum, and also, sometimes, one from the inferior surface of the brain. Small veins from the pons Varolii empty themselves into its an terior extremity.
The inferior petrosal sinuses also form an additional channel of communication between the lateral and cavernous sinuses. They are larger but shorter than the superior. In situ ation they correspond to the interval between the petrous bone and the occipital. They open into the inferior portion of the lateral sinus just before it unites with the jugular vein.
Transverse sinus. —This sinus establishes a communication between the petrosal and cavernous sinuses of opposite sides across the basilar process of the occipital bone. Some times there are two running parallel to each other. Cruveilhier states that the capacity of this sinus is much greater in old than in young subjects.
Cavernous .sinuses.—In point of shape these sinuses differ considerably from all the other sinuses of the dura mater. They are venous reservoirs, situated on each side of the sella Turcica, from which they are separated by the internal carotid arteries. Their name is derived from the spongy appearance which they present in their interior, owing to the existence of some filaments within them, which, by their inter lacement with each other, form a reticular texture there. It was formerly supposed that the carotid arteries lay in the cavity of these sinuses and were bathed by their blood; but it is easy to demonstrate by a little careful dis section that the inner membrane of the sinus adheres loosely to the outer wall of the artery, and that the sixth nerve passes betvveen them. In the outer vvall of each cavernous sinus there are channels for the reception of those nerves, which pass from the cranium into the orbit. These are the third nerve which is placed highest up, the fourth nerve which holds the next place, and the ophthalmic portion of the fifth. The cavernous sinus receives at its ante
rior extremity the ophthalmic vein, which col lects the blood from the eye-ball and other structures within the orbit, and which commu nicates also with the angular vein and with the frontal vein. (Hence the injected state of the vessels of the eye-ball when the brain is con gested, as in fever.) Veins from the inferior surface of the anterior lobes of the brain also open into it, also some from the middle lobe and from the dura mater. Posteriorly it com municates with both the petrosal sinuses, and veins from the cmnial bones open into its superior wall.
Circular sinus.—A communication is esta blished between the cavernous sinuses by means of the circular or coronary sinus which em braces the pituitary body, one portion lying in front of it and the other behind it, both open ing- by a common free orifice into the right and left cavernous sinuses. The posterior portion of the circular sinus is much larger than its anterior portion. Its size is much greater, according to Cruveilhier, in old subjects than in young ones. It receives small veins from the pituitary body, and also from the sphenoid bone and from the d ura mater.
It is impossible to exaniine this complicated arrangement of venous channels in con»exion with the dura mater of the brain without ad miring the beautiful provision which it affords against the undue accumulation of blood in the venous system within the cranium. In the fiNt place, we observe that these veins do not admit of dilatation beyond a prescribed extent, by reason of their being enclosed be tween layers of an inelastic and inextensible membrane. Next, we remark the safety pro vision which is afforded by the frequent com munication between them, so that if one chan nel were altogether closed or materialt- con tracted, there are many others by whit» the blood could return. Nor is a local congestion likely to take place to any extent, for such is the freedom of communication between the sinuses and the veins of the exterior of the cranium, that (all being devoid of valves) an overflow would readily he received by the latter without the least impediment. Lastly, we learn the great importance and value of local depletion as an agent for relieving vascu lar fullness within the head, owing to the free communication between the extra- and the intra-cranial circulation, and especially of the veins; and we may infer from anatomy that local depletion would most probably be more serviceable than general, for although the latter would diminish the amount of the mass of circulating fluid, it would not affect the relation between the venous and arterial systems, whilst it is evident that the former must affect the venous system more directly than the arterial. Moreover, the free communication between the circulation within and that without the cranium may explain somewhat the advantage that is often derived from the application of an intense cold to the external surface of the head.