490 HUMAN ANATOMY. 



laxity of the pericranium, and that their greater frequency in male children may 

 depend upon the larger size of the head in the male foetus. The close association of 

 the bloody effusion with the pericranium — an osteogenetic membrane — sometimes 

 results in the development of bone at the periphery of the swelling. The hard 

 ridge which is usually present at this situation may give rise, through contrast with 

 the relatively depressed centre, to the mistaken diagnosis of fracture of the skull. 



Occasionally a collection of blood beneath the pericranium communicates with 

 the diploic sinuses, when it will probably be situated to one side of the cranium ; or 

 with the superior longitudinal sinus, when it will be in the mid-line. No traumatic 

 history may be obtainable. The swelling will be soft, reducible, of varying tension, 

 and may receive from the brain a feeble pulsatile impulse. 



The importance of the emissary veins in transmitting extracranial infection to 

 the venous channels of. the dura may be mentioned here, but can better be under- 

 stood after the venous system has been described (page 876). 



2. The sicbapo7ieurotic connective tissue between the pericranium and the apo- 

 neurosis of the occipito-frontalis. This is so loose, thin, and elastic that the union 

 between these layers is not a close one. The motion of the "scalp" upon the skull 

 is a motion of the parts above upon the parts beneath this layer. Movable growths 

 will, therefore, be found to occupy the former region and immovable swellings will 

 probably have deeper attachments. Effusions of blood, suppuration, or infective 

 cellulitis occurring in the subaponeurotic space may extend widely, and may be 

 limited only by the attachments of the musculo-fibrous layer. They may reach, there- 

 fore, posteriorly to the superior curved line of the occipital bone, anteriorly to a little 

 above the eyebrows, and laterally to a level somewhat above the zygoma. Exten- 

 sive hsematomata are uncommon, as the vessels in this cellular tissue are few and 

 small. If they are large, they suggest fracture of the skull with laceration of a branch 

 of the middle meningeal artery or of a venous sinus. They may, however, by reason 

 of a hard border and soft centre, be mistaken for depressed fracture when the skull 

 itself is uninjured. 



Suppuration and cellulitis are often serious on account of the tendency to spread, 

 the possible extension to the meninges, and the difficulty in applying antisepsis, in 

 securing drainage, or, later, in obtaining the rest necessary for rapid healing. In 

 abscess the diffusion of the pus is favored by the density and the vitality of the super- 

 jacent layers, which, in consequence of the former property, do not soften and permit 

 pointing, and, because of the latter, do not slough and thus give exit to the pus, which 

 therefore may extend in the line of least resistance, — i.e., along the loose subapo- 

 neurotic layer. Wounds involving either the muscle or its aponeurosis, if transverse 

 to the direction of their fibres, gape widely. Their healing will be hastened by 

 firm bandaging of the whole cranium so as to control and limit the movements of 

 the scalp. 



3. The occipito-frontalis muscle and apo?ieurosis ; 4, xhe. superficial fascia ; 5, 

 the skin. .These three layers are so intimately blended that from the practical 

 stand-point they may be considered together. The thin aponeurosis is tied to the 

 skin (which is here thicker than anywhere else in the body) by dense, inelastic, 

 perpendicular and oblique fibres of connective tissue, enclosing little shot-like masses 

 of fat. This area is very vascular, almost all the vessels of the scalp being found in 

 it adherent to the cellular-tissue walls of the fat-containing compartments. As a 

 result of these anatomical conditions it is found that (i) suppuration is very limited 

 in extent ; (2) superficial infections (such as erysipelatous dermatitis) are accom- 

 panied by but little swelling ; (3) incised wounds do not gape ; (4) lacerated and 

 contused wounds are not followed by sloughing, which is also rare as a result of 

 continuous pressure, as from bandages ; (5) hemorrhage after wounds is abundant 

 and is persistent because of the adherence of the vessel- walls to the subcutaneous 

 layer of fascia, which prevents both their retraction and contraction ; (6) collections 

 of blood after contusions may, like the deeper ones already described, become very 

 firm at the periphery, — in this case from an excess of fibrinous exudate and from 

 the presence of particles of displaced fat, — while the inelastic fibres of cellular tissue 

 (from among which the fat particles have been driven out by the force of the blow) 

 remain depressed in the centre ; these appearances have not infrequently led to a 



