THYREOID IN TUETLES 285 



Topographical relations. As I have said above, the thyreoid in 

 Emys europaea lies within the large upward-curving arch formed 

 by the truncus innominatus, jast above the heart. As this 

 arch leans shghtly toward the right, the gland is not absolutely 

 in the middle, but is a little to the right. In front (on the ani- 

 mal's ventral side), the thyreoid is separated from the thoracic 

 wall by a lamellar connective, transparent and fairly tough, con- 

 sisting of several layers, continuous below with the pericardium 

 and surrounded abovQ by the large vessels of the neck. The 

 vascular arch along which the thyreoid lies is closely connected 

 with it, both by means of the vessels and by means of the connec- 

 tive tissue. The rear wall of the gland (toward the animal's 

 dorsal side) is in front of the trachea, with which it is not in con- 

 tact. It must be noted that in Emys europaea the trachea 

 divides into the two bronchi a little above the thyreoid, while in 

 Testudo graeca the division occurs much higher, near the base 

 of the tongue. 



Circulation and innervation. As in a man, the thyreoid in 

 these Chelonia is highly vascularized. The blood flows to it 

 through the two superior and the two inferior thyreoid arteries. 

 The inferior pair are short, but very capacious; they issue from 

 the truncus innominatus, and penetrate the gland at right angles, 

 passing through its outer inferior margin. Regarding the 

 behavior of the large vessels as they leave the heart, it should be 

 remembered that, whereas the left aorta reaches the left bronchus 

 without branching, the right aorta on the contrary, before 

 curvmg, sends off a large but very short trunk (truncus innomi- 

 natus) which forms a superior concavity and then divides into 

 the right and left carotid and subclavian arteries, after sending 

 off the inferior thyreoid arteries and the oesophageal arteries. 



The superior thyreoid arteries are longer but thinner; they 

 branch from the carotids and turn downward and inward, issuing 

 in the outer superior margin of the thyreoid gland. These 

 arteries (unUke the inferior thyreoid arteries which are always 

 present) are sometimes missing. 



It must be noted that the division of the truncus innominatus 

 mto subclavian and carotid sometimes occurs a little higher on 



