Under this heading will be considered the scalp, the bony sinuses, cranio-cerebral topography and the hypophysis.

The scalp

The importance of the scalp is best seen from an examination of its layers. These are:

1. skin;
2. subcutaneous fat and fibrous tissue;
3. the epicranius (occipito-frontalis) and aponeurosis;
4. the sub-aponeurotic layer of connective tissue;
5. the pericranium.

The first three layers are connected and move together. The thick skin supported by the dense fibrous subcutaneous layer and epicranial aponeurosis, is well adapted to protect the underlying skull from the effects of trauma, and in this connection the mobility of the first three layers on the subaponeurotic areolar tissue is important. A scalp wound does not gape widely unless it involves the epicranial aponeurosis, in which case it involves the subjacent "dangerous area" of the scalp, so-called because pus in this laj^er maj^ spread widely underneath the scalp and even give meningeal infection by spreading through the diploic or emissary veins. In the process of scalping (whether performed by the knife or by the hair being caught in machinery), separation takes place at this sub-aponeurotic layer which is loose, delicate and devoid of fat.

The numerous sebaceous glands frequently give rise to cysts in the scalp.

The epicranius and aponeurosis have been described elsewhere.

The pericranium differs from periosteum elsewhere in that it gives little nourishment to the bone beneath, which derives most of its blood-supply from the meningeal vessels. After necrosis of the skull there is no tendency to the formation of an involucrum of new subperiosteal bone as in the long bones. The pericranium is firmly adherent to the sutures of the skull bones, so that any subpericranial effusion of blood or pus is limited by the sutures.

Of the vessels of the scalp, the arteries, arising in the anterior region from the internal, in the posterior from the external, carotid, are peculiar in their position. Thus they he superficial to the deep fascia, which is here represented by the aponeurosis. From this position arises the fact that a large flap of scalp may be separated without perishing, as it carries its own blood-vessels. From the density of the layer in which the vessels run they cannot retract and are difficult to seize, haemorrhage thus being free. Finally, from their position over closely adjacent bone, ill-applied pressure may easily lead to sloughing. A practical point with regard to the veins is given below. The lymphatics from the front of the scalp drain into the anterior auricular and parotid, those behind into the posterior auricular, occipital and deep cervical nodes. The nerves are derived from all three divisions of the trigeminus, from the

facial (motor) and also from three branches of the second and third cervical. The supply from the fifth explains the neuralgia in acute iritis, glaucoma, and herpes frontalis, and also the pains shooting up from the front of the ear in late cancer of the tongue.

Section through the Scalp, Skull, and Dura Mater. (Tillaux.)

The emissary veins

These are communications between the sinuses within, and the veins outside, the cranium. Most of them are temporary, corresponding to the chief period of growth of the brain. Thus in early life, when the development of the brain has to be very rapid, owing to the approaching closure of its case, a free escape of blood is most essential, especially in children, with their sudden explosions of laughter and passionate crying.

The gravity of these emissary veins and their free communications with others are shown by the readiness Avith which they become the seat of thrombosis, and thus of blood-poisoning, in cranial injuries, erysipelas, infected wounds of the scalp, and necrosis of the skull. They include the following:

1. Vein through the foramen caecum, between the anterior extremity of the superior sagittal sinus and the nasal mucous membrane. The value of this temporary outlet is well seen in the timely profuse epistaxis of children. Other more permanent communications between the skuD cavity arid nasal mucous membrane pass through the ethmoid foramina. The fact that the nasal mucous membrane is loose and ill-supported on the nasal conchas ( turbinate bones) allows its vessels to give way readily, and thus forms a salutary safeguard to the brain, warding off many an attack of apoplexy.

2. Vein through the mastoid foramen, between the transverse (lateral) sinus and the posterior auricular and occipital veins. This is the largest, the most constant, and the most superficial of the emissary veins. Hence the old rule of applying blisters or leeches over it in cerebral congestion.

3. Vein through the posterior superior angle of the parietal between the superior sagittal sinus and the veins of the scalp.

4. Vein through the condyloid foramen between the transverse (lateral) sinus and the deep veins of the neck.

5. Vein through the hypoglossal canal between the occipital sinus and the deep veins of the neck.

6. Ophthalmic veins communicating with the cavernous sinus and the angular vein. These veins may be the source of fatal blood-poisoning, by conveying out of reach septic material, in acute periostitis of the orbit, or in osteitis, of dental origin, of the jaws.

7. Minute veins through the foramen ovale between the cavernous sinus and the pharyngeal and pterygoid veins.

8. Communications between the frontal diploic and supraorbital veins, between the anterior temporal diploic and deep temporal veins, and between the posterior temporal and occipital diploic veins and the transverse sinus. In addition to the veins specially mentioned, the scalp and sinuses communicate by numerous diploic veins, by those in the inter-sutural membrane, and thi'ough sutures before their obliteration, as already explained.

Structure of cranium

Two layers and intervening cancellous tissue. Each layer has special properties. The outer gives thickness, smoothness, and uniformity, and, above all, elasticity. The inner is whiter, thinner, less regular - e. g. the depressions for vessels. Pacchionian bodies, dura mater, and brain. The diploe, formed by absorption after the skull has attained a certain thickness, reduces the weight of the skull without proportionately reducing its strength, and provides a material which will prevent the transmission of vibrations.

A blow on the head may fracture the internal layer only, the external one and diploe escaping. This is difficult to diagnose, and thus it is impossible to judge of the severity of a fracture from the state of the external layer. This may be whole, or merely cracked, while the-internal shows many fragments, which may set up meningitis or other mischief. It is usual to find more extensive splintering of the inner than of the outer layer (table).

The average thickness of the adult skull-cap is about 5 mm. (0.2 in.). (Holden.) The thickest part is at the external occipital protuberance, where the bone is often 1.8 cm. (0.75 in.) in thickness. The thinnest part of the skull vault is over the temporal part of the squamous. The extreme fragility of the skull here is partly compensated for the by thickness of the soft parts; these two facts are always to be remembered in the diagnosis of a fracture of the skull here, after a slight injury. Other weak spots are the medial wall of the orbit, the cerebellar fossae, and that part of the middle fossa corresponding to the glenoid cavity.

Anatomical conditions tending to minimise the effects of violence inflicted upon the skull.

1. The density and mobility of the scalp.
2. The dome-like shape of the skull. This is calculated to bear relatively hard blows and also to allow them to glide off.
3. The number of bones tends to break up the force of a blow.
4. The sutures interrupt the transmission of violence.
5. The inter-sutural membrane (remains of fcetal periosteum) acts, in early life, as a linear buffer.
6. The elasticity of the outer layer (table).
7. The overlapping of some bones, e. g. the parietal by the squamous; and the alternate bevelling of adjacent bones, e. g. at the coronal suture.
8. The presence of ribs, or groins, e. g.
   1. from the crista galli to the internal occipital protuberance;
   2. from the root of the nose to the zygoma;
   3. the temporal ridge from orbit to mastoid;
   4. from mastoid to mastoid;
   5. from external occipital protuberance to the foramen magnum.
9. Buttresses, e. g. zygomatic processes and the greater wing of the sphenoid.
10. The mobility of the head upon the spine.