186 



Mr. Callender on the Thyroid Body. 



[1867. 



The Isthmus. 



In the dissections already referred to, the presence of a middle portion, 

 and its equal development with the lateral lobes, leads to the inference 

 that this central part is present from the earliest period, and that the thy- 

 roid isthmus is not formed by a growing together of two distinct side- 

 pieces. Yet in the human foetus, at four months, Fleischmann and Meckel 

 say that they found, as they described them, two lateral lobes only ; and 

 the hare, for example, has been written of as having but two distinct lateral 

 masses, as also were the Cetacea until Professor Turner* explained that the 

 thyroid of a well-grown male porpoise was a single mass extending across 

 the trachea, of which the median portion could hardly be described as an 

 intervening isthmus ; for in its supero-inferior diameter it equalled that of 

 the lateral portion. In addition, however, to the dissections already de- 

 tailed, I have examined the thyroid in fcetal hares, and have always found 

 the middle portion equally developed with the side lobes, and bounded by 

 notches, which seem to define it from them (fig. 5). With the growth 

 of this foetus, as also in the young of cats and of dogs, I have observed 

 that the central part appears to flatten and to lose the rounded lobular 

 condition, and sometimes it disappears altogether. 



The Lobes. 



Whilst, however, three lobes are chiefly indicated, lesser notches may 

 be occasionally seen, and continue to be noticeable as the foetus grows, 

 though they are very irregular and uncertain. Thus in a human foetus, 

 three inches and nine-tenths long, the left lobe (fig. 6)f is divided 

 into two portions by a deep fissure, one-half of it ascending to the left of 

 the middle line in front of the cricoid and thyroid cartilages, and there 

 are other notches faintly outlining a middle lobe. In a foetus four inches 

 and three-tenths long, the middle lobe is bifid, a cleft dividing it above ; 

 and in another foetus, four inches and six-tenths long, the entire thyroid 

 is very irregularly formed, broken into several lobes, but still showing at 

 its lower margin a division into three chief portions (figs. 7 & 8) J. 

 Here also a process ascends, budding out from the left side, tapers almost 

 to a point, and ends by being fixed to the under surface of the os 

 hyoides. 



Omitting lesser varieties, I will describe the following. The thyroid 

 from a foetus eight inches and seven-tenths long (fig. 10)§, consists 

 of two chief lobes, which meet, but are not united, in the middle line, 

 being separated by a deep fissure. From the left lobe, just anterior to 

 the lower angle of the thyroid cartilage, a small process projects upwards, 



* Transactions of the Eoyal Society of Edinburgh, vol. xxii. p. 320. 

 t This specimen is in the Museum of St. Bartholomew's Hospital. 

 i See also specimens Nos. 2 and o. 

 § See also specimen No. 6. 



