DEEP CERVICAL FASCIA. 291 



the opposite platysma in front of the symphysis, and even cross fr'om 

 the one side to the other below the chin, those of the right side over- 

 lapping those of the left ; and the posterior fibres are prolonged upon 

 the side of the face as far as the angle of the mouth, blending with the 

 triangularis oris and orbicularis oris muscles. The uppermost fibres 

 (nsorius, Santorini) liave been already described. 



VfJ7-h'fi/'>i.~- The muscular fibres of the platysma sometimes extend upwards on 

 the face and do^vnwards on the neck, shoulder and breast further than usual ; and 

 they occasionally take attachment to the clavicle. This muscle is the representa- 

 tive in man of a subcutaneous gi'oup of muscles, the paniiicuhix cartuK^Hn, largely 

 developed in most mammals, by which very varied movements of the skin and 

 some superficial parts may be given, as, for example, when the horse communi- 

 cates a rapid motion to the skin to free itself from insects, or the dog shakes oft" 

 the water after swimming, or the hedgehog elevates its spines. 



Nerves. — The platysma receives its principal motor nerves from the descend- 

 ing branches of the facial, but as this unites with the superficial cervical nerve 

 it may also be influenced tlu'ough some of the spinal nerves. 



Action. — The platysma, being much less developed in man than in animals has 

 a comparatively limited action in the human subject ; it raises the skin of the 

 neck into longitudinal wrinkles ; it also assists in drawing the angle of the jaw 

 downwards and outwards, and protects parts more deeply situated in tlie neck. It 

 is contracted in sudden fear, and assists in the expression of this emotion. 



MUSCLES AND FASCI2E OP THE NECK ANTERIORLY, 



Fascia. — The deep cervical fascia passes forwards from the anterior 

 border of the trapezius muscle over the sides and front of the neck 

 beneath the platysma myoides. Posteriorly it is continuous with the 

 layers of connective tissue with which the trapezius and deeper muscles 

 are invested ; it extends over the posterior triangle of the neck, viz., 

 the space bounded by the trapezius and sterno-mastoid muscles and 

 the clavicle : at the posterior border of the sterno-mastoid it divides 

 into two layers, which form an investment for that muscle ; these 

 unite again at the anterior border into a membrane which passes 

 forwards across the middle line, and covers the area bounded by the 

 middle line, the border of the jaw, and the sterno-mastoid muscle, and 

 •called the anterior triangle. In the posterior triangle the fascia is 

 attached inferiorly to the clavicle, and near that bone is perforated by 

 the external jugular vein, which in the previous part of its course lies 

 superficial to the membrane. In the anterior triangle it is bound 

 superiorly to the base of the jaw in front, and further back is con- 

 tinued over the masseter muscle {masseteric fascia), and the parotid 

 gland {iKirotid fascia) to the zygoma. 



In front the fascia is attached to the hyoid bone, and becoming 

 stronger as it descends, it splits, a little below the level of the 

 thyroid body into two distinct layers. Of these the more su])erficial 

 and weaker, running along the sterno-mastoid muscles, is fixed to the 

 sternum and the interclavicular ligament ; whilst the stronger layer, 

 lying under the former, and closely covering the sterno-hyoid and 

 sterno- thyroid muscles, is attached to the deeper surface of that bone. 

 These layers materially assist in closing the cavity of the chest behind 

 the sternum superiorly : between them there exists a quantity of loose 

 connective tissue and fat, and sometimes a small lymphatic gland. 

 Continuous with the deeper of those two layers, a fascia is found 



u 2 



