Cirsoid aneurysm is usually met with in the course of the temporal artery, and may involve the greater part of the scalp. Large, distended, tortuous, bluish vessels pulsating synchronously with the heart are seen and felt. They can be emptied by pressure, but fill up again at once on removal of the pressure. The patient complains of dizziness, headache, and a persistent rushing sound in the head. Ulceration of the skin over the dilated vessels, leading to fatal hæmorrhage, may take place.

They may be treated by excision, after division and ligation of the larger vessels entering the swelling; or the dilated vessels may be cut across at several points and both ends ligated. Krogius recommends the introduction of a series of subcutaneous ligatures so as to surround the whole periphery of the pulsating tumour, and interrupt the blood flow. Ligation of the main afferent vessels, or of the external or common carotid, has been followed by recurrence, owing to the free anastomatic circulation in the scalp. In some cases electrolysis has yielded good results.

Traumatic aneurysm of the temporal artery was comparatively common in the days when the practice of bleeding from this vessel was in vogue, but it is seldom met with now.

Arterio-venous aneurysm may also occur in the course of the temporal artery, as a result of injury, and is best treated by complete extirpation of the segments of the vessels implicated.

CHAPTER XII
THE CRANIUM AND ITS CONTENTS

Anatomy and Physiology.—The Cranium is irregularly ovoid in shape, and its floor is broken up by various projections to form three separate fossæ—anterior, middle, and posterior—in which rest respectively the frontal, the temporal, and the occipital lobes of the brain; the cerebellum, pons, and medulla oblongata also occupy the posterior fossa.

The outer table is the most elastic layer of the calvarium, and it varies greatly in thickness in different skulls and in different parts of the same skull. It is nourished chiefly from the pericranium which is firmly bound down along the lines of the sutures. The inner or vibreous table is thin and fragile, and its smooth internal surface is grooved by the middle meningeal and other arteries of the dura mater, and by the large venous sinuses. The intermediate layer—the diploë—is highly vascular, branches of the meningeal vessels anastomosing freely in its open porous substance with branches derived from the pericranial vessels. Some of its veins open into the external veins, and others into the intra-cranial sinuses, and they communicate with the emissary veins as these pass through the bone, which explains the spread of infective processes from the structures outside the skull to those within. The possibility of withdrawing blood from the interior of the skull by leeching, bleeding, or cupping depends on the existence of the emissary veins.

The Membranes of the Brain.—The dura mater is a fibro-serous membrane, the outer, fibrous layer constituting the endosteum of the skull, the inner, serous layer forming one of the coverings of the brain. Between the fibrous layer and the bone the meningeal vessels ramify; and along certain lines the two layers split to form channels in which run the cranial venous sinuses. Inside the dura, and separated from it by a narrow space—the sub-dural space—lies the arachno-pial membrane, consisting of an outer (arachnoid) layer which envelops the brain but does not pass into the sulci, and a highly vascular inner layer—the pia mater—which closely invests the brain and lines its entire surface.

The space between these layers—the sub-arachnoid space—is traversed by a network of fine fibrous strands, in the meshes of which the cerebro-spinal fluid circulates. Each nerve-trunk as it leaves the skull or spinal canal carries with it a prolongation of each of these membranes and their intervening spaces. The membranes gradually become lost in the fibrous sheaths of the nerves, and the sub-dural and sub-arachnoid spaces become continuous with the lymph spaces of the nerves.