296 THE MUSCLES AND FASCIAE OF THE HEAD AND NECK. 



from the skin and subcutaneous tissue over the upper part of the deltoid, pectoral, 

 and trapezius muscles ; thence they proceed upwards and inwards over the clavicle 

 and the side of the neck, gradually converging and approaching the muscle of the 

 opposite side. The greater number of the fibres are inserted into the outer surface 

 of the lower jaw from the mental protuberance to the attachment of the masseter ; 

 some of the inner fibres mingle with those of the opposite platysma in front of the 

 symphysis, and the innermost fibres of all cross from the one side to the other below 

 the chin, those of the right side being generally superficial, and are attached to- 

 the lower border of the jaw opposite the mental protuberance ; the posterior fibres 

 are prolonged upon the side of the face as far as the angle of the mouth, blending 

 with the depressor anguli oris and orbicularis muscles. 



Varieties, &c. The muscular fibres of the platysma sometimes extend upwards on the 

 face and downwards on the neck, shoulder and breast farther than usual ; and they occasion- 

 ally take attachment to the clavicle. The upper part of the muscle is occasionally joined by 

 a slip from the mastoid process, or from the occipital bone, and the frequently occurring- 

 muscular fasciculus known as the occi.p'tt<Hi* minor, spring-ing 1 from the fascia over the upper 

 end of the trapezius, and ending- similarly over the insertion of the sterno-mastoid, is. 

 probably, a less developed form of this accessory slip of the platysma. Suppression of the 

 whole platysma on one or both sides is recorded by Macalister, and of the lower half by 

 Geg-enbaur. This muscle is the representative in man of a subcutaneous sheet of muscle, the 

 panniculus carnosus, largely developed in most mammals, by which very varied movements of 

 the skin and some superficial parts may be effected, as. for example, when the horse com- 

 municates a rapid motion to the skin to free itself from insects, or the dog- shakes off the 

 water after swimming, or the hedg-ehog elevates its spines. 



Nerves. The platysma receives its principal motor nerves from the inframaxillary branch 

 of the facial, but as this unites with the superficial cervical nerve it may also be influenced 

 throug-h some of the spinal nerves. 



Action.. The platysma raises the skin of the breast and shoulder and draws the angle of 

 the mouth forcibly downwards and outwards ; at the same time it carries the skin of the 

 neck forwards, reducing the angle between the chin and neck. The muscle is put into action 

 in swallowing, and is also called into play in expressing- sudden terror. 



THE MUSCLES AND FASCLffi OF THE NECK ANTERIORLY. 



FASCIAE. The deep cervical fascia passes forwards from the upper border of 

 the trapezius muscle over the side and front of the neck beneath the platysma 

 myoides. Posteriorly it is continuous with the layers of connective tissue by which 

 the trapezius and deeper muscles are invested : it then 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 to the middle line, 

 where it becomes continuous with the fascia of the opposite side, 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 below to the clavicle, and near that bone is perforated by the external 

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

 membrane. In the anterior triangle it is bound above to the base of the jaw in 

 front, and farther back is continued superficially over the parotid gland (parotid 

 fascia) to the zygoma, sending upwards also a deep process between the sub- 

 maxillary and parotid glands, in which the stylo-maxillary ligament (p. 163) is 

 developed. 



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 superficial and weaker, running along the sterno- 

 mastoid muscles, is fixed to the anterior surface of the sternum ; while the 



