134 HEAD AND NECK 



floor or medial boundary of the triangle, and, if this view is 

 taken, the structures they cover, which lie more deeply, are 

 under cover of the floor. Those structures must now be 

 exposed. 



Dissection. Divide the anterior belly of the omo-hyoid along 

 the anterior border of the sterno-mastoid and turn it upwards 

 to its insertion into the hyoid bone. As that is done its twig of 

 supply from the loop called the ansa hypoglossi will be cut. 

 The ansa hypoglossi is formed by the union of the descending 

 branch of the hypoglossal nerve and the communicating branch 

 from the cervical plexus. Divide the sterno-hyoid as low down 

 as possible ; turn it upwards to its insertion into the body of the 

 hyoid bone and note its nerve of supply from the ansa hypoglossi. 

 Secure the nerve to the sterno-thyreoid from the ansa hypoglossi ; 

 then remove the fascia and expose the lower part of the thyreo- 

 hyoid muscle, the greater part of the sterno-thyreoid and the 

 anterior part of the thyreoid cartilage. Note that the sterno- 

 thyreoid is inserted into an oblique line on the outer surface of 

 the lamina of the thyreoid cartilage and that the thyieo-hyoid 

 springs from the same line and is inserted into the greater cornu 

 of the hyoid bone. The crico-thyreoid branch of the superior 

 thyreoid artery may be found passing downwards and forwards 

 along the upper end of the sterno-thyreoid, accompanied by the 

 external laryngeal nerve ; or the nerve and the vessel may lie 

 deep to the upper end of the muscle. 



Divide the sterno-thyreoid as low down as possible and turn 

 it upwards to its insertion ; remove the fascia under cover of it 

 and expose the lobe of the thyreoid gland, enclosed in its fascial 

 sheath. Below it, a small part of the side of the trachea will be 

 seen. 



The dissector should note that whilst the sterno-mastoid 

 remains undisturbed the posterior part of the lobe of the thyreoid 

 gland and its lower extremity are not exposed, but if the sterno- 

 mastoid is displaced backwards the whole of the lateral surface 

 of the lobe is brought into view. The dissector should note also 

 that, until the sterno-mastoid is displaced backwards, only a 

 small portion of the upper end of the common carotid and the 

 lower parts of the internal and external carotid arteries are 

 visible ; indeed, the common carotid may be entirely concealed. 

 Only a small part of the anterior border of the internal jugular 

 vein projects anterior to the sterno-mastoid in the upper angle 

 of the carotid triangle ; and it also is not uncommonly hidden 

 when the sterno-mastoid is well developed. During life, however, 

 when the muscle is soft and pliable the structures concealed by 

 it are readily exposed, for the muscle is easily displaced back- 

 wards after the fascia has been divided along its anterior border. 



In dissecting-room subjects, in which the muscles have been 

 hardened by formol, it is not possible to obtain a proper view of 

 the course and relations of the common carotid artery and the 

 internal jugular vein, or to appreciate the relations of the first 

 part of the subclavian artery and the relations of the scalenus 

 anterior muscle, until the sterno-mastoid has been reflected. 

 Divide the external jugular vein immediately below its origin by 



