This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The scalp is formed by the movable soft tissues which cover the skull. It is composed of three layers: skin, superficial fascia, and occipitofrontalis muscle with its aponeurosis. It is attached to the underlying pericranium by loose connective tissue called the subaponeurotic layer. The pericranium, or periosteum of the skull, is loosely attached to the bones by a small quantity of connective-tissue fibres called by some anatomists the subpericranial connective tissue. At the sutures, however, it is very firmly attached.
Fig 1. The scalp.
The principal affections of the scalp are wounds, inflammation, affections of the blood-vessels, tumors, and neuralgia. The peculiarities of these affections are determined by the anatomical structure of the parts.
The skin of the scalp is probably the thickest in the body, although not so dense as that of the heel. Besides the hair, it contains abundant sweat and sebaceous glands. These latter are connected with the hair-follicles and are near the surface. The skin increases in thickness from the frontal to the occipital region.
The superficial fascia consists of a net-work of connective-tissue fibres which run from the skin above to the aponeurosis of the occipitofrontalis below. In its meshes are fat, blood-vessels, nerves, and lymphatics. The hair-bulbs often pierce the skin and extend into this layer.
The fibres of the superficial fascia bind the skin so firmly to the aponeurosis beneath that when the skin is moved the aponeurotic layer is carried with it. The arrangement of the fibres is shown in Fig. 3. Fibres starting from the point A not only pass directly down to B, but also to each side to the points C and D. In the same way, fibres starting from B not only pass upward to A, but also forward to E and backward to F. Now, if the skin is moved in the direction of the forward arrow, the fibres E B and A D are tightened and drag the aponeurosis forward. If the skin is moved in the direction of the backward arrow, the fibres A C and FB are tightened and so drag the aponeurosis backward. Thus it is seen that the aponeurosis must follow the movements of the skin.
Fig. 2. - Layers of the scalp.
Merkel describes the epicranial aponeurosis as dividing into two layers, one inserting into the skin and the other into the rim of the orbit (Hand. der top. Anat. Bd. 1, p. 17). The bellies of the muscle are comparatively short, about 5 cm. in length, the remaining tissue extending between them constituting the aponeurosis. As it comes downward from the temporal ridge, over the sides of the head, the aponeurosis becomes thinner and gives attachment by its superficial surface to the anterior and superior auricular muscles. It then proceeds downward to be attached to the upper edge of the zygoma.
Fig. 3. - Diagram illustrating the method of attachment of the skin to the aponeurosis of the occipitofrontalis muscle.
Fig. 4. - Showing now the periosteum in childhood dips between the bones in the line of the sutures.
Contraction of the occipitofrontalis muscle causes the skin of the forehead to wrinkle transversely. It is a muscle of expression, and blends with the pyramidalis nasi and corrugator supercilii. It is supplied by branches of the facial nerve.
The subaponeurotic tissue is very loose and abundant, so that it does not tend to confine the movements of the scalp, but favors them. Hence the scalp is readily torn loose from the skull in scalping, machinery accidents, etc. This tissue is so loose that effusions accumulate here and spread extensively. It contains only a few blood-vessels.
The pericranium in its normal condition is a thin, tough membrane containing few blood-vessels. Except at the sutures, where it is firmly attached and dips down between the bones, it is comparatively easily stripped from the skull and does not convey much nourishment to it. It is deficient in osteogenetic or bone-forming properties, so that when it is raised off the skull in operations, and the bone removed from beneath, as occurs in trephining, fractures, etc., new bone is not produced.
The subpericranial tissue is so scanty and loose, particularly in infancy, that it readily allows the pericranium to be raised and effusions to occur beneath.