378 BULLETIN OF THE BUREAU OF FISHERIES. 



one might expect to find instances of remotely placed deposits. We are therefore 

 surprised to find the sharp delimitation to the region already described which appears to 

 exist in our wild specimens, with one marked exception, i. e., the jugular pit to which 

 reference has already been made. It has been assumed by some writers that aberrant 

 thyroid deposits in some regions might be frequently expected. This has been due to 

 the development of tumor nodules in regions beyond the normal and usual seat of 

 distribution, such as the lower mouth parts and gill arch region. The only outlying 

 tumors of this sort which may be inferred with certainty to arise from original deposits 

 of thyroid are the so-called pit tumors. 



In the adult trout there occurs on the ventral side of the head between the dentary 

 bones a median irregular depression or blind pit open to the exterior. It is the region 

 where the muscles between the dentaries and those of the branchiostegals become 

 common. The skin dips into its ramifications with many plications and infoldings. 

 It is an entirely superficial and exterior landmark, and though independent of and well 

 removed from the thyroid region in the adult, is of some importance in thyroid pathology, 

 since at an early stage it is in closely apposed relation to the thyroid region, and is the 

 occasional seat of detached thyroid follicles and of independent thyroid tumors. This 

 much branched and partitioned cavity may be designated as the jugular pit (fig. n). 

 It may be recognized at or soon after the hatching of the embryo. Normal thyroid 

 has been several times observed immediately beneath the pit epithelium in wild brook 

 trout (fig. 14), where its presence may be readily explained by the mechanics of devel- 

 opment of the parts (fig. 12). Since of 91 trout with tumors, 25 showed pit tumors, one 

 may conclude that more than one-quarter of all brook trout have normally some thyroid 

 follicles in the region of the pit. 



Occasionally detached tumors develop in the gill arches. We have never actually 

 seen thyroid follicles on the free portions of the gill arches, and there is little embryo- 

 logical presumption in favor of such deposits, notwithstanding the relation of the gill 

 arches with the thyroid region. The tip of the lower jaw has even much less anatomical 

 relation to the thyroid region, rendering it an unlikely place for stray follicles. The 

 examination by serial sections of the tips of the lower jaws of about 25 trout failed to 

 show any thyroid here. The region is called into question as a seat of normal thyroid 

 deposit on account of the rare cases of a thyroid tumor occupying this site. The 

 actually observed occurrence of normal thyroid in the pit and the incidence of tumors 

 here make it extremely probable that no other extralimital deposits occur with any 

 frequency, else such would declare themselves in tumor formation. 



HISTOLOGY. 



A closed alveolus is the unit of the thyroid gland. These alveoli are apparently 

 independent of each other save as they are more or less bound together by the vascular 

 framework and connective tissue stroma. Their lumina have no connections with 

 each other and each alveolus independently discharges its secretion into the circulation. 

 Many alveoli are quite isolated from their kind and lie. loosely in the tissue, whether 

 connective, muscle, bone, cartilage, or fat. In the simplest adult condition (fig. 14-21) 



