THE ANATOMY OF THE THYROID GLAND 175 



The left thyroid artery usually supplies a greater portion of the 

 gland than the right (fig. 3), though the relative area is subject to 

 extreme variation and in occasional instances the ratio may be 

 reversed (fig. 4). The area of distribution in the great majority 

 of individuals is approximately as indicated in fig. 3. 



THE HYPOBRANCHIAL CIRCULATION AND THE ORIGIN OF THE 



THYROID VESSELS 



In attempting to trace the circulation of the thyroid gland by 

 means of injection experiments I was at once struck with the diffi- 

 culty of reaching the gland by means of injections into the gill 

 arteries, the ventral aorta or the heart. It was obvious that the 

 thyroid artery has no direct connection with the ventral aorta or 

 the afferent branchial vessels, a fact which seems to have been first 

 observed by Simon ('44) who stated, without further explanation 

 orany outline of his reasons therefor, that the thyroid gland in Raia 

 ''never receives the smallest share of supply from the branchial 

 artery with which it is in contact." This fact seems to have been 

 seldom recognized and never sufficiently emphasized by later 

 writers. 



The thyroid artery arises either from the mandibular artery, or 

 by a common trunk with this artery, from an arterial sinus at the 

 ventral extremity of the first branchial cleft (figs. 5 and 6) ; this 

 sinus forms the ventral portion or connecting vessel of the efferent 

 vascular loop contained in the hyoidean hemibranch and first holo- 

 branch. From the dorsal extremity of this same loop the first 

 efferent branchial artery passes to the dorsal aorta. It is there- 

 fore necessary for fluid injected into the ventral aorta or its imme- 

 diate branches to pass through the gill capillaries before it can enter 

 the thyroid arteries, and few injection fluids readily pass through 

 capillary vessels. On the other hand, fluid injected into the thy- 

 roid artery or, as I later found, into any portion of the hypobran- 

 chial system passes readily into the vessels of the thyroid gland; 

 such injections I repeatedly made, into the sinus at the ventral 

 end of the first efferent branchial loop, into the thyroid artery, the 

 median hypobranchial artery, and even into the coronary artery 



