734 HUMAN ANATOMY. 



line for the skin incision. The artery is usually tied below the digastric muscle — 

 i.e., in the superior carotid triangle (page 548) — and between the origins of the supe- 

 rior thyroid and the lingual arteries — because that is the longest interval without 

 branches. After the skin, superficial fascia, platysma, and deep fascia have been 

 divided, the anterior edge of the sterno-mastoid cleared and drawn outward at the 

 lower portion of the wound, and the facial, lingual, or superior thyroid veins — if they 

 present — ;drawn aside or tied and cut, the posterior belly of the digastric muscle 

 above should be identified. Just beneath it the hypoglossal nerve crosses the artery, 

 and a little lower — about the middle of the incision — the tip of the greater cornu of 

 the hyoid bone may be felt. At this level — above the origin of the superior thyroid 

 and below that of the lingual — the artery lies just to the inner side of the internal 

 carotid (but somewhat superficial to it) and of the internal jugular vein, and has the 

 superior laryngeal nerve in close relation behind it. 



The internal carotid has been tied at this level by mistake for the external 

 carotid. To avoid this it should be remembered that the external carotid (<z) is 

 more anterior ; (^b) is more superficial ; (<:) is usually smaller, especially in the 

 young ; (a?) gives off branches ; (<f) is close to the tip of the hyoid bone ; \f) is in 

 contact with the hypoglossal nerve ; and ( g') that compression of the isolated vessel 

 arrests the temporal and facial pulses. The needle is passed from without inward to 

 avoid the internal jugular. » 



The ligature has been applied just below the parotid gland — i.e.^ above the 

 digastric muscle. The incision (on the same line) should extend from the lobe of 

 the ear to the hyoid bone, the sterno-mastoid should be drawn outward, the poste- 

 rior belly of the digastric and the stylo-hyoid muscle downward, and the parotid 

 upward and inward. 



The collateral circulation is carried on from the cardiac side through {a) the 

 branches of the opposite external carotid ; {b) the inferior thyroid on the affected 

 side ; (r) the branches of the ophthalmic of the same side ; and (t^) the profunda 

 cervicis, anastomosing respectively with (a) the branches of the ligated external 

 carotid ; {b) the superior thyroid ;• (r) the facial (from the same vessel — the external 

 carotid) ; and {^d ) the princeps cervicis. 



I. The Superior Thyroid Artery. — The superior thyroid artery (a. thy- 

 roidea superior) (Fig. 692) arises from the anterior surface of the external carotid, a 

 short distance above its origin, and is at first directed almost horizontally anteriorly, 

 but soon turns downward and, passing over the superior laryngeal nerve and beneath 

 the omo-hyoid and thyro-hyoid muscles, breaks up into a number of branches which 

 enter the substance of the thyroid gland. It possesses always a calibre of consider- 

 able size, but varies directly according to the size of the gland, and inversely 

 according to the amount of blood reaching the gland from other sources. It anas- 

 tomoses abundantly with its fellow of the opposite side and with the inferior thyroid 

 branch of the subclavian. 



Branches — From its horizontal portion are given off — 



(a) An infrahyoid branch (rannis hjoideus), which passes along the lower border of the 

 hyoid bone, supplying the muscles inserting into that bone. 



(<$i) A sterno-mastoid branch (ramus sternocleidomastoideus), always small and occasionally 

 wanting, which passes downward and backward across the sheath enclosing the common 

 carotid to enter the substance of the sterno-cleido-mastoid muscle. 



(r) A superior laryngeal branch (a laryngea superior), which passes forward and downward 

 beneath the thyro-hyoid muscle and, piercing the thyro-hyoid membrane along with the superior 

 laryngeal nerve, is distributed to the intrinsic muscles and mucous membrane of the laryn.x. 



From its descending portion it gives off — 



{d) The crico-thyroid branch (ramus cricothyroideus). usually of small .size, which passes 

 horizontally forward over the crico-tii\'roid membrane and anastomoses with its fellow of the 

 opposite side, giving off branches which perforate the membrane and are distributed to the 

 muscles and mucous membrane of the lower part of the laryn.x. 



Variations. — The superior thyroid may give origin to both the ascending pharyngeal and 

 the ascending palatine. The crico-thyroid not infrequently arises from the superior laryngeal, 

 and may appear to be the main stem of that artery, and the superior laryngeal may arise directly 

 from the external carotid. 



