THE HEAD AND NECK, POSTERIOR. 139 



It also extends downward to the cartilage of the first rib, 

 this portion forming the loop by which the tendon and 

 posterior belly of the omohyoid muscle is kept in position. 



Between layers number one and two is left a triangular 

 space Burns' space that has no width above, but below 

 is represented by the space between the posterior margins 

 of the sternomastoid. This space contains fat, lymphatic 

 glands, the anterior jugular vein, and connective tissue to 

 keep them in position. 



It is also stated that the second layer is continued under 

 the clavicle, splits to enclose the subclavius muscle, and 

 reunites to form the costocoracoid membrane. That the 

 sheath for the subclavian vein is derived from this second 

 layer, while for the artery it comes, as stated below, from 

 the third layer. 



(3) The third layer of the cervical fascia. This invests 

 the carotid vessels, arteries, and internal jugular vein, pneu- 

 mogastric nerve, thyroid gland, trachea, oesophagus, and 

 many other structures. This is also given off from the 

 under surface of the first layer behind the sternomastoid 

 muscle. It passes forward, splitting to enclose the internal 

 jugular vein, pneumogastric nerve, carotid arteries (forming 

 their sheaths), thyroid gland (forming its capsule), the 

 trachea, and oesophagus, and finally unites in the middle 

 line with corresponding layers from the other side. 



Besides the structures above enumerated this layer en- 

 sheaths all the structures found between the under surface 

 of the second layer and in front of the prevertebral (fourth) 

 layer. 



Above, this third layer passes up to the base of the skull 

 and is attached to the basilar and jugular processes of the 

 occipital bone, the petrous process of the temporal bone, the 

 spinous and pterygoid processes of the sphenoid bone. 



