This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Fractures of the skull are almost always produced by violent contact of the skull with some solid body. In some cases the fracture is produced by a blow from a moving body, as when a person is struck by a club. In others, the skull is moving and strikes a body at rest, as when a person falls and strikes the head on a pavement. It is not necessary to discuss in detail the mechanism of fractures of the skull; it is sufficient to state that nearly all fractures start from the point of impact and radiate to distant regions. The effect of fracturing blows on the skull of a child is different from their effect on the skull of an adult.
A child's skull is thin and weak, and while, to a certain extent, fragile is more flexible than that of an adult. It is on this account that blows are more liable to expend their force locally, at the point of impact, and not produce fractures at a distance. Therefore, it follows that fractures of the base are rare in children in comparison with fractures of the vault. Extensive fissured fractures are also rare. A blow will crush the skull of a child at the point of impact, much as an egg-shell is broken at one spot by hitting it with a knife handle. A marked example of this was seen in the case of a small boy who, while playing, was struck by a baseball on the left frontal eminence. A distinct circular depression or cup was produced exactly corresponding to the shape of the ball. There were no symptoms of cerebral concussion, because the force of the blow was expended on the bone and not transmitted to the brain within. As pointed out by Mr. Rickman Godlee, the sutures in very young children being soft, the transmission of the force from one bone to another is prevented.
The diploic structure of the skull is not well developed until adult age, therefore the bone is homogeneous. It is also elastic, and, particularly in infants, it may be dented without being seriously fractured; these dents are apt to disappear and become level with the surrounding bone as the child grows older. The dura mater is more adherent in children and fractures are, on that account, more liable to tear it and even lacerate the brain beneath.
As adult life is reached the inner and outer tables of the bones become separated, leaving the space between to be filled by the diploic tissue. The diploŽ consists of cancellous bone in the meshes of which run the diploic veins and capillaries. Both the inner and outer tables are brittle, but the inner especially so. It is also harder and more compact than the outer table. On account of this difference we find in cases of fracture that the inner table is more comminuted than the outer, so that, while the outer may show a single line of fracture, the inner table immediately beneath may be broken into several fragments. This is one reason why trephining is so frequently resorted to.
In rare cases there may be depression of the inner table with none of the outer. A case of this kind occurred during our Civil War. A soldier, while looking over a rampart, was struck a glancing blow by a bullet, on the upper anterior portion of the skull. The outer table at the site of injury was not at all depressed, but the inner table had a large piece broken off, which injured the membranes.
The elasticity of the skull is shown in cases of fracture in which hairs are found imbedded in the line of fracture. Figure 21 is from such a case. A negro was struck on the head by a falling rock and an extensive longitudinal fracture was produced in which many hairs were fastened. About a centimetre from the main fracture was a small fissure, not over a centimetre long, and sprouting up out of it, like bushes from the bare ground, were a number of hairs. In such cases the hairs are carried into the line of fracture by the force of the blow; the elastic bone then springs back into place and pinches the hairs, thus holding them in place.
The bones of the adult skull are very strong and firmly fixed. The sutures begin to unite at the age of forty years and are likely to have disappeared at the age of seventy. Even in young adults the fibrous tissue between the bones has so nearly disappeared that they practically act in transmitting force as one continuous bone. For these reasons slight blows do not cause fractures. It takes a very heavy blow usually to cause a fracture and the force is so great that shock or concussion of the brain with disturbance of its functions is a common symptom.
The force of the blow is expended first at the point of impact, and if a fracture occurs it usually starts there. From that point it radiates to other portions of the skull, so that fractures of the vault frequently extend to the base. The course pursued by the fracture has been formulated into a law by Aran; that they take a straight line from the point of impact on the vault to the base of the skull, and are not deflected by the sutures.
Charles Phelps ("Traumatic Injuries of the Brain") found that in 127 cases of fracture of the base of the skull, 12 implicated the base only. So that, if we are able to say that there is a fracture of the base of the skull, there are over 10 chances to one of its extending up into the vault. In only two were the fractures more than a slight fissure; so that in a marked fracture of the base there would be 63 chances to one of its extending into the vault. Also, from Aran's law, we see that, if we diagnose a fracture through the middle ear, we may be pretty sure that the force was applied directly above, and be led to trephine accordingly.
Fig. 21. - Small piece of the skull showing hairs imprisoned in a linear fracture - actual size.
A man fell from an electric light pole and was brought to the hospital with bleeding from the ear and other symptoms of fracture of the skull. He became wildly delirious, and, feeling sure that the fracture of the base was an extension from the vault, although no depression could be felt, he was trephined above the external auditory meatus and a large epidural effusion of blood evacuated. He recovered and resumed his work. In this case, as soon as the bone was exposed, a thin line of fracture was seen running down to the base in the region of the external ear.