SCALP, in anatomy, the covering of the top of the head from the skin to the bone.
The skin of the scalp is thick and contains a large number of hair follicles. The arteries are remarkable for their tortuosity, which is an adaptation to so movable a part ; for their anastomos ing across the middle line with their fellows of the opposite side, an arrangement which is not usual in the body; and for the fact that, when cut, their ends are held open by the dense fibrous tissue in which they lie, so that bleeding is more free in the scalp than it is from arteries of the same size elsewhere in the body.
The veins do not follow the twists of the arteries but run a straight course ; there is often a considerable distance between an artery and its companion vein. Accompanying the veins are the larger lymphatic vessels. From the forehead the lymphatics accompany the facial vein and usually reach their first gland in the submaxillary region, so that in the case of a poisoned wound of the forehead sympathetic swelling or suppuration would take place below the jaw. From the temple the lymphatics drain into a gland lying just in front of the ear, while those from the region behind the ear drain into glands lying close to the mastoid process. In the occipital region a small gland (or glands) is found about a third of the distance from the external occipital protuberance to the tip of the mastoid process (see SKULL).
The nerve supply of the scalp in its anterior part is from the fifth cranial or trigeminal nerve (see NERVE, Cranial) ; behind the ear the scalp is supplied with sensation by the great auricular and the small occipital (see NERVE, Spinal), while behind these, and reaching as far as the mid line posteriorly, the great occipital is distributed.
Beneath the skin and fibrous tissue lies the epicranium, formed by the two fleshy bellies of the occipito-frontalis muscle and the flattened tendon or aponeurosis between them. Of these two bellies the anterior (frontalis) is the larger, and, when it acts, throws the skin of the forehead into transverse puckers. The much smaller (occipitalis or posterior) belly usually merely fixes the aponeurosis for the frontalis to act, though some people have the power of alternately contracting the two muscles and so wag ging their scalps backward and forward as monkeys do.
Deep to the epicranium is a layer of very lax areolar tissue con stituting a lymph space and allowing great freedom of movement to the more superficial layers; it was this layer which was torn through when a Red Indian scalped his foe. So lax is the tissue here that any collection of blood or pus is quickly distributed throughout its whole area, and, owing to the absence of tension as well as of nerves, very little pain accompanies any such effusion.
The deepest layer of the scalp is the pericranium or the external periosteum of the skull bones. This, until the sutures of the skull close in middle life, is continuous with the Jura mater which forms the internal periosteum, and for this reason any subperi cranial effusion is localized to the area of the skull bone over which it happens to lie. Moreover, any suppurative process may extend through the sutures to the meninges of the brain.
(F. G. P.)