482 HUMAN ANATOMY. 



upward and inward and are inserted into the body of the mandible from the sym- 

 physis to the insertion of the masseter, the more posterior fibres extending upward 

 upon the face towards the angle of the mouth and becoming lost partly in the fascia 

 of the cheeks and partly among the muscles of the lips. 



Nerve-Supply. — By the inframandibular branch of the facial nerve. 



Action. — The contraction of the platysma results in drawing the lower lip 

 downward and outward and at the same time raising the skin of the neck from 

 the underlying parts. It is one of the most important muscles employed in the 

 expression of horror and intense surprise. It does not seem probable that the 

 muscle has much eflect in producing depression of the mandible, an action which 

 it might be expected to possess on account of its upper attachment. 



Relations. — The platysma rests upon the deep fascia of the neck and covers 

 all the structures at the front and sides of that region. Upon its deep surface lie the 

 external jugular vein, the superficial lymph-nodes of the neck, and the superficial 

 branches of the cervical plexus. It covers also the sterno-mastoid muscle and the 

 depressors of the hyoid bone, and, above, the digastric and mylo-hyoid muscles, 

 together with the submaxillary gland and the lower portion of the parotid. 



Variations. — There is usually more or less decussation of the two muscles across the 

 median line, especially in their upper parts, where, indeed, a certain amount of decussation may 

 be considered a normal condition. The muscle is subject to considerable amounts of variation 

 in its development, sometimes forming a very thin, pale layer largely interspersed with connective 

 tissue, and at other times it is composed of strong, deeply colored bundles with much less inter- 

 mixture of connective tissue. Its extension upon the face may also vary considerably, some- 

 times being traceable as high up as the zj-goma and extending backward to behind the ear. On 

 the other hand, it may be very considerably reduced in size, especially below, a complete absence 

 of the lower half of the muscle having been observed. 



2.. Occipito-Frontalis (Fig. 499). 



Attachments. — The occipito-frontalis ( m. epicranius) is a muscular and aponeu- 

 rotic sheet which covers the entire vertex of the skull from the occipital region to the 

 root of the nose. It consists of two muscular portions, one of which, the occipitalis, 

 arises from the superior nuchal line and inserts after a short course into the posterior 

 border of the epicranial aponeurosis, while the other, the frontalis , taking its origin 

 from the anterior border of the galea, is inserted into the skin in the neighborhood 

 of the eyebrows, over the glabella, and into the superciliary arches, a portion of it 

 being frequently prolonged downward upon the nasal bone, forming what has been 

 termed \\\& pyramidalis nasi (m. procerus), which is frequently described as a distinct 

 muscle. 



The epicranial apojieurosis (galea aponeurotica) (Fig. 499) is a dense aponeu- 

 rotic sheet which covers the entire vertex of the cranium and is prolonged laterally 

 over the temporal fascia as a thin layer which extends almost to the zygoma. On 

 its superficial surface it is intimately associated with the integument, being united to 

 its deeper surface by a thin but close and resistant layer of fascia which represents 

 the superficial fascia of other regions of the body and in which are embedded the 

 vessels and nerves of the scalp. The under surface of the galea is, however, smooth, 

 and is connected with the periosteum by a lax layer of connective tissue, so that it is 

 capable of considerable movement to and fro upon the periosteum, the skin being 

 carried with it in such movements. A section through the scalp at the vertex would 

 show from without inward (i ) the skin, (2) the dense superficial fascia with its vessels 

 and nerves, (3) the epicranial aponeurosis, (4) loose connective tissue, and (5) 

 periosteum v'Fig. 504). 



Nerve-Supply. — The occipitalis is supplied by branches from the posterior 

 auricular branch of the facial, the frontalis by branches from the rami temporales of 

 the same nerve. 



Action. — The occipitalis acting alone will draw backward the galea aponeurotica, 

 while the frontalis draws it forward. If, however, the galea be fixed by the occipitales, 

 the action of the frontales is to raise the eyebrows and throw the skin of the forehead 

 into transverse wrinkles, both of these actions being greatly increased by the simul- 

 taneous contraction of both the occipitales and the frontales. It is consequently the 



