THE ANATOMY OF THE THYROID GLAND 189 



peculiar to deserve passing mention. It may be described as con- 

 sisting of two triangles whose bases are fused in the median line, 

 the apices directed outward, the anterior borders convex and con- 

 forming to the anterior margin of the basi-hyal cartilage, the 

 posterior borders concave and free, except for their attachment to 

 the deep cervical fascia. In the median line the conjoined bases 

 are prolonged backward to form a short median projection; 

 anteriorly a shallow notch separates the two lateral triangular 

 halves. The gland is approximately bilaterally symmetrical 

 (figs. 1, 3 and 4). 



The thin, almost membranous character of the gland in Muste- 

 lus canis offers an excellent opportunity for the recognition of a 

 lobar or lobular structure if such exists, for the whole gland 

 is frequently no more than four or five follicles in thickness and 

 may be stained, cleared and mounted in toto, giving very excel- 

 lent microscopical pictures of the entire organ. I have not been 

 able to find any indication of definite lobes or lobules. Portions 

 of the thyroid substance are here and there wanting, as observed 

 by Lombard ('09), and these deficient areas occur more frequently 

 in the posterior than in the anterior half of the gland. In one of the 

 thirty-two fishes of this species the deficiencies were so great that 

 the gland was only represented by a few specks which were posi- 

 tively identified as portions of the thyroid only after microscopical 

 examination. A similar case was found in Squalus, and one gland 

 from Carcharias consisted of three small pieces. 



Occasionally the posterior border presents a notched deficiency 

 in or near the median line. There may be one, two or three such 

 notches, either symmetrically or asymmetrically disposed. Defici- 

 encies of the thyroid tissue also occur within the gland and may, 

 or may not, be connected with the notches in the posterior border. 

 These deficiencies are all of inconstant occurrence, irregular loca- 

 tion, and could scarcely be taken to indicate any suggestion 

 of definite lobes. They seem rather to be due to the extreme 

 thinness of the gland and in many of the thicker specimens they 

 are in no way indicated. When present they are occupied by con- 

 nective tissue continuous with the glandular capsule. Frequently 

 they transmit the larger thyroid vessels. 



