254 MUSCLES AND FASCIAE. 



the Platysma, the external jugular vein may be seen descending from the angle of 

 the jaw to the clavicle. It is essential to remember the direction of the fibres of 

 the Platysma, in connection with the operation of bleeding from this vessel ; for 

 if the point of the lancet is introduced in the direction of the muscular fibres, the 

 orifice made will be filled up by the contraction of the muscle, and blood will not 

 flow ; but if the incision is made in a direction opposite to the course of the fibres, 

 they will retract, and expose the orifice in the vein, and so facilitate the flow of 

 blood. 



Relations. By its external surface, with the integument to which it is united 

 closely below, but more loosely above. By its internal surface, below the clavicle 

 which it covers, with the Pectoralis major, Deltoid, and Trapezius. In the neck, 

 with the external and anterior jugular veins, the deep cervical fascia, the super- 

 ficial cervical plexus, the Sterno-mastoid, Sterno-hyoid, Omo-hyoid, and Digastric 

 muscles. In front of the Sterno-mastoid, it covers the sheath of the carotid ves- 

 sels ; and behind it, the Scaleni muscles and the nerves of the brachial plexus. 

 On the face, it is in relation with the parotid gland, the facial artery and vein, and 

 the Masseter and Buccinator muscles. 



The deep cervical fascia is exposed on the removal of the Platysma myoides. 

 It is a strong fibrous layer, which invests the muscles of the neck, and incloses 

 the vessels and nerves. It commences, as an extremely thin layer, at the back 

 part of the neck, where it is attached to the spinous processes of the cervical 

 vertebras, and to the ligamentum nuchaa ; and, passing forwards to the posterior 

 border of the Sterno-mastoid muscle, divides into two layers, one of which passes 

 in front, and the other behind it. These join again at its anterior border ; and, 

 being continued forwards to the front of the neck, blend with the fascia of the 

 opposite side. The superficial layer of the deep cervical fascia, that which passes 

 in front of the Sterno : mastoid, if traced upwards, is found to pass across the 

 parotid gland and Masseter muscle, forming the parotid and masseteric fasciae, 

 and is attached to the lower border of the zygoma, and more anteriorly, .to the 

 lower border of the body of the jaw ; if the same layer is traced downwards, it is 

 seen to pass to the upper border of the clavicle and sternum, being pierced just 

 above the former bone for the external jugular vein. In the middle line of the 

 neck, the fascia is thin above, and connected to the hyoid bone ; but it becomes 

 thicker below, and divides, just below the thyroid gland, into two layers, the more 

 superficial of which is attached to the upper border of the sternum and inter- 

 clavicular ligament ; the deeper and stronger layer is connected to the posterior 

 border of that bone, covering in the Sterno-hyoid and Sterno -thyroid muscles. 

 Between these two layers is a little areolar tissue and fat, and occasionally a 

 small lymphatic gland. The deep layer of the cervical fascia, that which lies 

 behind the posterior surface of the Sterno-mastoid, sends numerous prolongations, 

 which invest the muscles and vessels of the neck ; if traced upwards, a process of 

 this fascia, of extreme density, passes behind and to the inner side of the parotid 

 gland, and is attached to the base of the styloid process and angle of the lower 

 jaw, forming the stylo-maxillary ligament; if traced downwards and outwards, it 

 will be found to inclose the posterior belly of the Omo-hyoid muscle, binding it 

 down by a distinct process, which descends to be inserted into the clavicle and 

 cartilage of the first rib. The deep layer of the cervical fascia also assists in 

 forming the sheath which incloses the common carotid artery, internal jugular 

 vein, and pneumogastric nerve. There are fibrous septa intervening between 

 each of these parts, which, however, are included together in one common invest- 

 ment. More internally, a thin layer is continued across the trachea and thyroid 

 gland, beneath the Sterno-thyroid muscles ; and at the root of the neck this may 

 be traced, over the large vessels, to be continuous with the fibrous layer of the 

 pericardium. 



The Sterno-deido-mastoid (fig. 153) is a large thick muscle, which passes 

 obliquely across the side of the neck, being inclosed between the two layers of 

 the deep cervical fascia. It is thick and narrow at its central part, but is broader 



