CARCINOMA OF THE THYROID IN SALMONOID FISHES. 4OI 



junction and in the gill spaces on both sides, protruding between the first and second 

 branchial arches, and penetrating the floor of the mouth in the median line at the junction 

 between the second and third branchial arches. 



Microscopically, this extensive growth is found to infiltrate the surrounding struc- 

 tures, bone, cartilage, and muscle. It may be classified as alveolo-tubular type. In 

 many regions it presents a characteristic solid structure. Only in occasional areas do 

 a few follicles contain poorly staining colloid. In the region of the third branchial arch 

 is found a cross section of bone, in the lumen of which is a group of thyroid follicles of 

 strictly normal appearance. (Fig. 47.) The follicles are of the usual size filled with 

 homogeneously and deeply staining colloid, the epithelium is flattened, protoplasm 

 small in amount, the nuclei stain deeply and homogeneously. A study of the preceding 

 and succeeding serial sections fails to show any opening through the wall of bone. The 

 tumor tissue is in immediate contact with the shell of bone for a considerable extent of 

 its circumference, the remainder is covered with periosteum and dense connective tissue. 



The significance of this finding is that here we have a large tumor of tubulo-alveolo- 

 solid type infiltrating the surrounding structures, while lying within the medullary 

 space of the bone structure and included in the tumor tissue is a deposit of normal 

 thyroid tissue which by its appearance, and the presence of deeply stainable colloid, 

 must be in physiological continuity with the metabolism of the fish. That the deposit 

 of normal thyroid within the bone has remained unaltered because it was protected from 

 an agent working from without by being inclosed within the bone is probable, but no less 

 important is the fact that it shows clearly that portions of the thyroid structure in the 

 Salmonidae may undergo malignant change, while other portions of the thyroid tissue 

 remain strictly normal in appearance and retain their physiological function until, it 

 may be said, as this tumor was very advanced, the last stages of the disease. 



A great deal has been said about the significance of the capsule of the thyroid in 

 mammals. Here the evidence of malignancy consists in a breaking through the capsule 

 and infiltration of the surrounding structures. As the thyroid structure of the teleost 

 has no capsule, such a criterion can not be applied, and some have ascribed the infil- 

 trative characteristics of these tumors to the isolated character of the follicles and lack 

 of a delimiting capsule. One of the best evidences of malignancy in the mammalian 

 thyroid is the development of isolated nodules of malignant character within the struc- 

 ture of an enlarged thyroid, and here the evidence of infiltration has not to do with a 

 capsule, but the infiltration of surrounding thyroid structure. A determination of 

 exactly analogous conditions in our tumors indicates that we have in the proliferation 

 of the tumor tissue in the Salmonidae an expression of genuine malignancy. 



Infiltration of bone and cartilage. The growth of these extensive tumors naturally 

 leads to the erosion and destruction of cartilage and bone. The mere presence of 

 thyroid tissue within the bone spaces of the branchial arches is in itself of no signifi- 

 cance. Deposits of normal thyroid tissue are often encountered in the open spaces of 

 both bone and cartilage. The shape of the bony and cartilaginous structure in the 

 floor of the mouth in the teleost is frequently such that the thyroid tissue grows in 

 through small openings in the base of the arches. In many instances, however, we 



