234 HEAD AND NECK 



posterior part of the hyoglossus and the middle and inferior 

 constrictors of the pharynx. The two latter muscles cannot 

 be displayed at present, but the thyreo-hyoid is exposed below 

 the great cornu of the hyoid bone, and part of the hyoglossus 

 can be seen in the angle between the great cornu of the hyoid 

 and the lower part of the posterior belly of the digastric. 



The Muscular Triangle. When the deep fascia which 

 covers the muscular triangle is removed portions of the 

 three muscles are brought into view. Postero-superiorly is 

 the anterior belly of the omo-hyoid, more anteriorly and on 

 the same plane is the sterno-hyoid, and below and anterior to 

 the latter, but on a deeper plane, is a small part of the sterno- 

 thyreoid. 



The muscles mentioned may be considered to form the 

 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. These 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 this is done its twig of supply from the ansa hypo- 

 glossi will be cut. 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 loop which is called the ansa hypoglossi, and is formed 

 by the union of the descending branch of the hypoglossal nerve and the 

 communicating branch from the cervical plexus. 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 thyreo-hyoid springs from the 

 same line and is inserted into the great cornu of the hyoid bone. The 

 crico-thyreoid branch of the superior thyreoid artery may be found passing 

 downwards and anteriorly 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 lateral 

 lobe of the thyreoid gland, and below it a small part of the side of the 

 trachea. 



The dissector should note that whilst the sterno-mastoid remains undis- 

 turbed the posterior part of the lateral lobe of the thyreoid gland and its 

 lower extremity are not exposed, but if the sterno-mastoid is displaced 

 posteriorly 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 

 posteriorly 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 



