PRACTICAL CONSIDERATIONS : THE NECK. 



551 



usually described as part of {d) a. process — tracheal — which leaves the sterno-mastoid 

 nearer its anterior border, and, runnings behind the sterno-hyoid and sterno-thyroid 

 muscles, descends in front of the trachea and the thyroid gland to become connected 

 with the fibrous layer of the pericardium. 



The adhesion of the deep fascia to the blood-vessels, by preventing contraction 

 and collapse of their walls, favors hemorrhage and increases the risk of the entrance 

 of air into divided veins. 



Tracing the layers of fascia vertically and from the surface inward, it will be 

 useful to remember that the superficial layer (a, Fig. 546) passes to the top of the 

 sternum (sending a slip to be attached to its posterior border) and to the clavicle. 

 The second layer (d) descends behind the depressors and in front of the thyroid 

 gland and trachea to merge into the pericardium, and farther out to form a sheath for 

 the omo-hyoid and for the subclavian vein, and is lost in the sheath of the subclavius. 



Fig. 545. 



Trachea 



Fusion of superficial layer in mid-line 



Space 3a 

 Thyroid body 



CEsophagus. 



Carotid artery. 



Internal jugular vein 

 Vertebral vessels 



Space I 

 Extern, jugular vein 



Spinal nerves, cut 

 obliquely 



Spinal cord 



Trapezius muscl 



Vertebral 



Space a^ 

 b 



Fascia covering 

 posterior triangle 



Fascia passing 

 beneath trapezius 



Ligamentum niichae 



Section across neck at level of seventh cervical vertebra. 



This relation of the omo-hyoid is of value in enabling that muscle, when the 

 hyoid is fi.xed, to increase the tension of this layer of fascia, and thus hold open and 

 prevent atmospheric pressure upon the walls of the vessels — especially the veins — 

 and the soft parts (including the pulmonary apices) at the base of the neck. 

 Hilton uses this function of the muscle — which connects it with the act of respiration — 

 to illustrate the precision of the nerve-supply to muscles generally. The omo-hyoid 

 arises in close proximity to the suprascapular notch, and therefore to the supra- 

 scapular nerve. Yet it never receives a filament from that nerve, but is supplied 

 by the cervical nerves to bring it in relation to the movements of the other neck 

 muscles, is connected with the hypoglossal to associate it with the movements of the 

 tongue, and with the pneumogastric to enable it to act as above described during 

 forced respiration, when the rush of air into the thorax might otherwise cause harmful 

 increase of atmospheric pressure in the lower cervical or supraclavicular region. 



The pretracheal layer is found between the depressors and the trachea passing 

 down to its pericardial insertion. Hilton thus explains this insertion : ' ' The peri- 

 cardium is most intimately blended with the diaphragm, distinctly identified with it, 

 and capable of being acted upon by it at all times. It is also attached above to the 

 deep cervical fascia. It is thus kept tense by the action of the respiratory muscles 

 in the neck attached to the cervical fascia above and the diaphragm attached to it 



