The principles and practice of modern surgery

by Roswell Park

The young and the aged have no distinction of tables of the skull, but the diploë which separates the two tables is an affair of middle age, develops slowly, and disappears after the same fashion—sometimes to such an extent as to leave the skull of almost paper-like thinness. In all operations, then, upon the young and the old the surgeon should proceed with extreme caution, as if expecting to find the skull quite thin. The lower limit of the squamous bone proper is the so-called mastosquamosal suture, and operations confined to the squamous plate alone are safe from injuring the sigmoid sinus on its inner side. The ridge at the posterior root of the zygoma indicates, by its lower border, the level of the mastoid antrum. A few lines above this is the level of the base of the brain. The mastoid is present at birth and appears externally by the second year. Its antrum is present also at birth, though its air cells do not develop until after puberty, their location being previously occupied by cancellous tissue. Most of these cells open into the antrum, a few directly into the tympanum. They are not always separated from the sigmoid sinus by bone. The partition between them is perforated by minute veins, forming an easy communication between the sinus and the antrum. Air escaping from the mastoid cells into the overlying tissue may cause emphysema from a basal fracture. In all operations upon the mastoid antrum the operator should keep to its outer side, and the higher and the more closely to the posterior zygomatic ridge he makes the first opening the more sure is he to escape injuring the facial nerve. The groove for the sigmoid sinus extends to the jugular foramen from a point on the outside corresponding to the asterion. The lateral sinus may be indicated externally by a line from the superior border of the mastoid to the inion—i. e., from the asterion to the inion.

The frontal sinuses are usually separated by a septum, which is often incomplete or wanting. They are variable in size and outline, and do not develop until after the seventh year. The infundibulum, by which they empty into the nasal cavity, is often so small that when the lining membrane is involved it becomes closed, and retention, with its accompanying symptoms—pain, tenderness, swelling, etc.—may ensue. Ulceration and erosion, however, may cause perforation internally through the supra-orbital plates, so that pus may penetrate through the inner half of the orbit.

Aside from its direct communication the superior longitudinal sinus connects with the basal sinuses through the middle cerebral and the Sylvian veins, while communications with the middle meningeal veins are abundant. Where the frontal and diploëtic veins enter the longitudinal sinus there frequently are dilatations in which marasmic thromboses often originate. This sinus is also connected with the veins of the nasal septum, so that a septic phlebitis may be propagated from the nose. So much of the lateral sinus as is contained in the sigmoid groove is known as the sigmoid sinus, which connects directly with the exterior through the mastoid and the posterior condyloid veins. In sinus thrombosis this mastoid vein is likewise affected. One or more condyloid veins accompany the hypoglossal nerve through the anterior condyloid foramen, and may also serve for the propagation of infection or exit of pus.

While septic particles may be carried from any part of the lateral or sigmoid sinuses—usually through the internal jugular—they may also be carried by way of the other veins above mentioned or the occipital sinus, all of which empty directly into the subclavian without passing through the internal jugular. These sinuses are all rigid tubes, always open, while the veins are thin and flexible, their caliber constantly varying with inspiration and expiration. The sinuses contain no valves, and these are very rare in the cerebral veins.

So far as the lymphatics are concerned there is free and easy communication between the internal and external plexuses and nodes. Into the superficial nodes, along the external jugular, outside of the deep fascia, empty all the external lymphatics of the head. Intracranial infection shows itself in swelling of the deep cervicals beneath the deep fascia. Lymphatics are abundant in the dura, and pathogenic organisms, once housed within the dura, find it easily open to invasion. The potential interval between the dura and the arachnoid is termed the subdural space, when considerable effusion may occur without marked symptoms, owing to its easy diffusion, while blood here poured out may travel even to the lowest parts of the spine and cause death by pressure upon remote points.

The arachnoid bridges over the convolutions and does not extend into the sulci. It is not vascular; at certain points it is adherent to the pia, at others it does not touch it. The subarachnoid space is formed in the latter way, and within it most of the cerebrospinal fluid is contained. This space is unevenly distributed over the brain surface, most prominently beneath the posterior two-thirds of the brain, where there is a wide interval between the arachnoid and the pia, extending forward around the medulla and pons and as far forward as the optic nerves. This space connects with the ventricles by the foramen of Magendie, as well as with the sheaths of the cranial nerves. Where these nerves escape from the brain or cord they are covered by all three membranes, the layers being most distinct along the optic nerves. Fluid injected into the subdural space may pass along the spinal nerves as far as the limbs. It is essential to realize this in order to appreciate how extensive is the surface exposed in leptomeningitis.

Internal hydrocephalus is often the result of closure of the foramen of Magendie. The cerebrospinal fluid is rapidly reproduced after traumatic escape. External hydrocephalus or accumulation in the subarachnoid space, is a condition frequently due to tuberculous infection.

The pia is the vascular coat of the brain, supplied with an extensive network of fine nerve fibers derived from the sympathetic and the cranial nerves, having intimate relations with the brain, to such an extent that leptomeningitis and encephalitis are almost inseparable. The nerve supply to the cerebral membranes explains the severe pain of meningitis.

INJURIES TO THE SOFT PARTS OF THE CRANIUM.

In direct connection with what has been stated above it is well to emphasize that the venous communications between the exterior and interior of the cranium are numerous, and that the frequency of these anastomoses explains the ease with which extracranial infections are propagated within; in other words, these explain the frequency of septic mischief in the brain after external injuries.