This is a flat, thin, digastric muscle, extended from the occiput to the forehead (from which circumstance its name is derived), and placed immediately beneath the cranial integument, to which it closely adheres, at the same time that it rests upon the arch of the skull, over which it slides. It consists of two broad but short fleshy bellies, united by an intervening aponeurosis.
The occipital part
The posterior fleshy portion is attached, by short tendinous fibres, to the external two-thirds, sometimes much less, of the superior curved line of the occipital bone, and to the mastoid portion of the temporal, bone immediately above the Sterno-mastoid muscle. The fleshy fibres which are from an inch to an inch and a halth in length, proceed upwards and inwards, and terminate in distinct white tendinous fibres, which soon become continuous with the aponeurosis. Between the occipital muscles of opposite sides of the head there is a considerable, but in different cases a varying, interspace, which is occupied by the epi-cranial aponeurosis.
The fleshy fibres, of which this portion of the muscle is composed, extend downwards and forwards on the frontal bone; they are longer and broader than those of the occipital part, but the fibres are paler and less distinctly muscular; their upper margin, being their junction with the aponeurosis, presents a curved line, which is a little below the coronal suture; the inner fibres, corresponding with the median line, descend vertically, and become continuous with the pyramidalis nasi; 5 the middle fibres, longer than the others, terminate by becoming blended with the orbicularis, 4 and corrugator supercilii ; and the external fibres curve inwards somewhat, and become blended with those of the orbicularis palpebrarum over the external angular process. The inner margins of the right and left frontal muscles are blended together for some space above the root of the nose.
The aponeurosis of the occipito-frontalis (membrana epicrania ; galea aponeurotica capitis) extends over the upper surface of the cranium uniformly from side to side, without any separation into lateral parts.
It must therefore be regarded as a single structure, having connected with it the occipital and frontal muscular strata above described, and at the same time uniting the muscles of one side with the other, and combining their action.
Posteriorly, in the interval between the occipital parts of the muscles, the aponeurosis is fixed to the occipital protuberance and curved line above the trapezius; in front it presents in the middle an angular elongation, which intervenes for a short distance between the margins of the frontal muscles before they join; laterally, it has connected with it the superior and anterior auricular muscles. In the situation of the temporal ridge it loses the aponeurotic character, and is continued over the temporal fascia to the zygoma by a layer of laminated cellular membrane. The fibres are chiefly longitudinal, following the direction of the muscles; and they will be found distinctly tendinous where they receive the fibres of the occipital portions of the muscle.
The aponeurosis is firmly connected with the skin and subcutaneous granular fat (in which several blood-vessels and nerves ramify) by numerous short fibro-cellular bands; and it adheres loosely to the subjacent pericranium, through the medium of thin cellular membrane devoid of fat. Hence the muscles, when thrown into action, move the integuments with the aponeurosis (the hairy scalp) on the immediate investment of the skull. Hence, too, while they together admit of being easily and speedily stripped from the calvarium, the skin cannot be separated from the aponeurosis and muscle with facility. The integument is likewise closely connected with the frontal portion of the muscle, and the skin of the forehead is, in consequence, folded or wrinkled when this contracts.
Some anatomists consider the whole to be a four-headed muscle, having two fleshy portions behind, and two in front, all connected by a single layer of aponeurosis, which rests on the cranium. Others vi£w it in a different way, the fleshy parts being taken as separate muscles, and named from their position, the anterior one being the "frontal" muscle, the posterior the " occipital."
All the muscular parts having one broad common aponeurosis, they act together : their first effect is to draw up the eyebrows, the next to throw the skin of the forehead into transverse folds or wrinkles; and to move the hairy scalp backwards and forwards, by bringing the occipital and frontal parts of the muscle alternately into action.