Over the temporal region the aponeurosis of the occipito-frontalis becomes very attenuated, losing itself gradually as it approaches the zygoma by numerous small insertions into the temporal fascia. It also sends some prolongations into the subcutaneous tissues, but in this region the aponeurosis is not nearly so intimately associated with the subcutaneous tissues as in the scalp proper.
The temporal muscle takes origin from, and is covered in by, the temporal fascia, which arises from the temporal ridge and runs down to be inserted into the zygoma, splitting as it does so to enclose branches of the temporal and orbital arteries, embedded in fat. The fascia is remarkably strong and dense, so as to be practically indistinguishable from bone on palpation. Were an abscess occurring in the temporal region, this fascia would tend to prevent its pointing locally, and would direct it under the zygoma, whence it may extend even to the neck.
The pericranium in this region is much more adherent than it is over the vault, and hence subpericranial haematomas are very unlikely to occur.
The bones composing the cranial vault are developed in membrane, possess few osteoblasts, and have but little healing power. Thus, after destruction of a portion of the vault, it is unusual to find repair by osseous tissue, the bone generally being replaced by fibrous tissue.
Necrosis most often affects the frontal and parietal bones, and not infrequently the external table alone is affected. Extensive destruction of the anterior portion of the vault is sometimes due to syphilitic ulceration, the dura presenting at the bottom of the ulcer. Craniotabes is a condition met with in early life, affecting generally the parieto-occipital region, due to rickets or inherited syphilis, in which the bone becomes thin and parchment-like.
The inner table of the skull is thinner and more brittle than the outer, and in fractures or gun-shot wounds involving both tables is generally much more extensively shattered than the outer. In some cases where a rifle-bullet strikes the skull tangentially, cutting a groove in the external table, the inner, although apparently not directly implicated, has been shattered over a considerable area and driven into the brain substance. Sometimes also, as the result of injury, the inner table may be fractured without evidence of fracture affecting the outer table. The inner table is grooved by the sinuses and the Pacchionian bodies, and also by the middle meningeal artery. In some cases the artery is actually embedded in the bone, and is then particularly liable to injury from fracture.
The diploic tissue between the two tables is very vascular, most of the blood being derived from the meningeal vessels. The return flow of venous blood is chiefly directed through the diploic veins toward the great sinuses, but a portion is carried by emissary veins to the superficial blood channels, which thus bring superficial and deep systems into direct communication. Other emissary veins run directly from the sinuses to the superficial veins.
These emissary veins are of great importance surgically, as they afford access for pyogenic organisms to the sinuses and meninges from superficial affections at these parts. The most important are : (a) Mastoid, which runs from the sigmoid sinus to the posterior auricular or occipital veins through the mastoid foramen ; (b) parietal, from the superior longitudinal sinus to the scalp veins through the parietal foramen ; (c) superior orbital, communicating with the ophthalmic and facial frontal diploic veins ; (d) vein of foramen caecum, connecting those of the nasal mucous membrane with superior longitudinal sinus ; (e) veins from cavernous sinus through the foramen ovale ; (/) veins from the lateral sinus throug 1 the postcondylar foramen, etc., to deep occipital veins ; (g) numerous small vessels running from inside to outside of the skull through the sutures ; (h) frontal, nasal, and angular veins, with the cavernous sinus, through the ophthalmic veins.
The skull varies greatly in thickness-not only in different individuals, but also at different parts-a point which must be kept in mind when trephining. Speaking generally, it is thickest over the frontal and occipital regions, attaining a maximum at the posterior occipital protuberance, and is thinnest over the squamous portion of the temporal.
The frontal sinuses are formed at the expense of the diploic tissue, which is absent at these places, while the tables are wide apart. They vary much in size in different individuals, and are generally larger in males. The septum between them is frequently displaced to one side or other, and sometimes only one sinus may exist, or they may be altogether absent.
They cap generally be examined in a darkened room by transillumination from a lamp in the mouth. When normal, they are fairly translucent, as a rule, whereas they become opaque when filled with pus.
The cranial sutures are of importance surgically, as their arrangement, interdigitation, and complicated structure, while producing practically a single bone, yet do much in conjunction with the elasticity of the bones themselves to modify fracturing forces. At birth the persistence of the anterior fontanelle (which normally closes by the second year) increases the adaptability of the head. The posterior fontanelle normally closes before birth. Separation at the sutures rarely occurs apart from fracture save in early youth, but a few cases of separation at the squamous suture have occurred. The coronal and sagittal sutures have been most frequently affected by fracture. The skull also possesses a series of buttresses, by which the force of blows is transmitted and diffused, thereby minimizing the chances of fracture. Sometimes, however, a blow struck upon a strong, unyielding bone, such as the occipital, may be transmitted to a weak bone-e.g., the orbital plate of the frontal-with such force as to fracture it, the bone struck remaining intact (fracture by contre-coup). In old persons the sutures tend to disappear, synostosis occurring, while the bones themselves lose their elasticity to a great extent, fracture of the skull being then more easily produced.