Vascular System of the Thyroid Gland. 489 



This description of the thyroid blood-supply must be taken only 

 as indicating a general scheme. Certain variations will, of course, 

 be noted. The size of the lobules, depending upon the number of 

 follicles composing it, will, of course, vary and with it the number 

 of arterial branches supplying it. Also the blood-supply of the 

 individual follicles is subject to certain variations. In many cases 

 a single follicle, as shown in Fig. 9, besides receiving its blood-supply 

 from what might be termed its own follicular artery, receives small 

 branches from an artery which supplies an adjacent follicle. 



The veins while in general following the course of the arteries, 

 also show many variations. Often the vein which springs from the 

 follicular network, instead of passing back side by side with the 

 artery, empties into a vein which follows the course of an artery 

 supplying follicles on the far side of the lobule. Such a picture is 

 seen in Fig. 6. The capillary network surrounding the follicle 

 anastomoses very commonly with that of an adjacent follicle. 



Thus it is seen, that in the thyroid, too, we have a definite 

 system of blood-supply, a definite system of vascular units, which 

 repeat themselves with a greater or less constancy throughout the 

 entire organ. These vascular units correspond in most instances 

 very closely with the structural units. 



The smallest vascular unit present is the follicular unit, which 

 consists of the follicular network, each in the case of the dog and 

 man, with its own artery and vein. In the cat, as already stated, 

 this network is not so rich and distinctive, and follicular arteries and 

 veins are not present. Yet, the large mesh containing the follicle 

 is the homologue of the network and may be regarded as the small- 

 est unit present. This vascular unit corresponds to the histological 

 unit of the individual follicle. 



The next vascular unit in size is the lobular unit. This is com- 

 posed of (1) the arteries of the fourth order which run over the 

 clumps of follicles having as their direct branches the follicular ar- 

 teries, and (2) the arteries of the third order which pass between 

 the lobules. This vascular unit corresponds to the structural unit 

 of the lobule. 



