THE ANATOMY OF THE THYROID GLAND 171 



THE THYROID VESSELS 



The anterior margin of the thyroid gland is firmly attached 

 by a dense fibrous fold of fascia to the antero-ventral margin of 

 the basi-hyal cartilage. Its ventral surface is in contact with 

 the coraco-mandibular muscle, to which it is firmly united by 

 a fascia. Its dorsal surface rests upon the basihyal cartilage and 

 the insertion of the coraco-hyoid muscle as described in the 

 preceding section. The lateral angles and posterior margin of 

 the thyroid gland are continuous with a fold of the deep cervical 

 fascia which is placed between the coraco-mandibular and coraco- 

 hyoid muscles, loosely attached to the opposed surfaces of these 

 muscles, but more firmly fixed to their lateral margins, so as to 

 form an aponeurosis for each. This fold of the fascia is much 

 thickened anteriorly where it approaches the margin of the thyroid 

 gland: at this point it incloses a transverse anastomosing vein 

 which connects laterally with the hyoid sinuses. 



At the posterior margin of the gland the fascia splits, a thin 

 layer passing dorsally between the gland and the coraco-hyoid 

 muscle, a thicker portion extending forward over the ventral sur- 

 face of the organ, between it and the coraco-mandibular muscle. 

 This ventral division is of special importance; it contains the largo 

 thyroid sinus consisting of an intricate net of veins and lymphatics, 

 which connects laterally with the hyoid sinus and in the median 

 line spreads over the ventral surface of the thyroid gland so that 

 when distended with blood it entirely obscures the organ. 



The thyroid sinus is surrounded with connective tissue con- 

 taining a network of lymphatic vessels. Ink injected in the 

 living animal into the space between the lateral margin of the 

 thyroid sinus and the ventral end of the first branchial cleft 

 will after a few minutes be found filling many of the lymphatic 

 vessels of the thyroid sinus as well as many other perivascular lym- 

 phatics in relation with most of the cervical veins and the arteries 

 of thehypobranchial system (fig. 8). The lymphatics of the thyroid 

 plexus are so mumerous and anastomose so freely that when 

 filled with ink they form a sac-like investment entirely obscuring 

 the sinus and the thyroid gland (fig. 2). An attempt to inject with 



