120 THE SKELETON. 



as well as membranes, it is termed an enceplwlocele ; and when the protruded brain is a prolonga- 

 tion from one of the ventricles, and is distended by a collection of fluid from an accumulation in 

 the ventricle, it is termed an hydrencephalocele. This latter condition is frequently found at the 

 root of the nose, where a protrusion of the anterior horn of the lateral ventricle takes place 

 through a deficiency of the fronto-nasal suture. These malformations are usually found in the 

 middle line, and most frequently at the back of the head, the protrusion taking place through 

 the fissures which separate the four centres of ossification from which the tubular portion of the 

 occipital bone is originally developed (see page 61). They most frequently occur through 

 the upper part of the vertical fissure, which is the last to ossify, but not uncommonly through 

 the lower part, when the foramen magnum may be incomplete. More rarely these protrusions 

 have been met with in other situations than those two above mentioned, both through normal 

 fissures, as the sagittal, lambdoid, and other sutures, and also through abnormal gaps and 

 deficiencies at the sides, and even at the base of the skull. 



Fractures of the skull may be divided into those of the vault and those of the base. Frac- 

 tures of the vault are usually produced by direct violence. This portion of the skull varies in 

 thickness and strength indifferent individuals, but, as a rule, is sufficiently strong to resist a very 

 considerable amount of violence without being fractured. This is due to several causes : the 

 rounded shape of the head and its construction of a number of secondar} 7 elastic arches, each 

 made up of a single bone ; the fact that it consists of a number of bones, united, at all events in 

 early life, by a sutural ligament, which acts as a sort of buffer and interrupts the continuity of any 

 violence applied to the skull ; the presence of arches or ridges, both on the inside and outside of 

 the skull, which materially strengthen it ; and the mobility of the head upon the spine which 

 further enables it to withstand violence. The elasticity of the bones of the head is especially 

 marked in the skull of the child, and this fact, together with the wide separation of the indi- 

 vidual bones from each other, and the interposition between them of other softer structures 

 renders fracture of the bones of the head a very uncommon event in infants and quite young 

 children ; as age advances and the bones become joined, fracture is more common, though still 

 less liable to occur than in the adult. Fractures of the vault may, and generally do, involve the 

 whole thickness of the bone ; but sometimes one table may be fractured without any correspond- 

 ing injury to the other. Thus, the outer table of the skull may be splintered and driven into the 

 diploe, or in the frontal or mastoid regions into the frontal or mastoid cells, without any injury 

 to the internal table. And on the other hand, the internal table has been fractured, and por- 

 tions of it depressed and driven inward, without any fracture of the outer table. As a rule, in 

 fractures of the skull the inner table is more splintered and comminuted than the outer, 

 and this is due to several causes. It is thinner and more brittle; the force of the violence as it 

 passes inward becomes broken up, and is more diffused by the time it reaches the inner table ; 

 the bone, being in the form of an arch, bends as a whole and spreads out, and thus presses the 

 particles together on the convex surface of the arch i. e. the outer table and forces them 

 asunder on the concave surface or inner table ; and, lastly, there is nothing firm under the inner 

 table to support it and oppose the force. Fractures of the vault may be simple fissures or starred 

 and comminuted fractures, and these may be depressed or elevated. These latter cases of 

 fracture with elevation of the fractured portion are uncommon, and can only be produced by 

 direct wound. In comminuted fracture a portion of the skull is broken into several pieces, 

 the lines of fracture radiating from a centre where the chief impact of the blow was felt ; 

 if depressed, a fissure circumscribes the radiating line, enclosing a portion of .skull. If 



as a poker. 



Fractures of the base are most frequently produced by the extension of a fissure from the 

 vault, as in falls on the head, where the fissure starts from the part of the vault which first 

 struck the ground. Sometimes, however, they are caused by direct violence, when foreign 

 bodies have been forced through the thin roof of the orbit, through the cribriform plate of the 

 ethmoid from being thrust up the nose, or through the roof of the pharynx. Other cases of 

 fracture of the base occur from indirect violence, as in fracture of the occipital bone from impac- 

 tion of the spinal column against its condyles in falls on the buttocks, knees, or feet, or in cases 

 where the glenoid cavity has been fractured by the violent impact of the condyle of the lower jaw 

 against it from blows on the chin. 



The most common place for fracture of the base to occur is through the middle fossa, and 

 e fissure usually takes a fairly definite course. Starting from the point struck, which is 

 generally so-mewhere in the neighborhood of the parietal eminence, it runs downward through 

 the parietal and squamous portion of the temporal bone and across the petrous portion of this 

 bone, frequently traversing and implicating the internal auditory meatus, to the middle lacerated 

 foramen *rom this it may pass across the body of the sphenoid, through the pituitary fossa to 

 the middle lacerated foramen of the other side, and may indeed travel round the whole cranium 



u ^u c m Pletely separate the anterior from the posterior part. The course of the fracture 

 should be borne in mind, as it explains the symptoms to which fracture in this region may "ive 



ise ; thus, it the hssure pass across the internal auditory meatus, injury to the facial and 

 auditory nerves may result, with consequent facial paralysis and deafness; or the tubular pro- 



ongation of the arachnoid around these nerves in the meatus may be torn, and thus permit of 

 the escape of the cerebro-spmal fluid should there be a communication between the internal ear 



