566 



ANATOMY OF VERTEBRATES. 



438 



elongate form. In Bears the thyroids are joined by a long slender 

 band at their lower ends. In Felines the uniting band appears 

 to become longer and more slender by age, and sometimes dis- 

 appears. Cuvier notes three distinct connecting bands in a Civet- 

 cat, 1 and two such bands in a Marmoset monkey. In the Aye- 

 aye the thyroid bodies, elongate, triangular, and flattened, lie 

 upon the sides of the second to the seventh tracheal rings in- 

 clusive, and are devoid of connecting transverse strip. 2 In most 

 Quadrumana the thyroids are united, but by a longer and narrower 

 band or ' isthmus ' than in Man. In him the thyroid bodies are 

 not only relatively large, but are united by an 'isthmus' so broad 



as to usually extend across 

 two or more upper rings of the 

 trachea; moreover, a process 

 extends from the upper part 

 of the isthmus, as the e py- 

 ramid ' or ( mesial column,' 

 which in some subjects reaches 

 to the hyoid bone. Many 

 varieties have been noted in 

 the human thyroid. Some- 

 times the isthmus is absent, 

 as normally in certain lower 

 Mammalia ; and sometimes 

 there is more than one pyra- 

 midal or ascending process. 



§ 352. Thymus.— This body 

 is distinguished from the thy- 

 roid by its wide central ca- 

 vity, and by its diminution of 

 volume or disappearance after 

 early age. In the Human sub- 

 ject, e.g., at birth the thymus, 

 fig. 438, a, a, may weigh 240 

 grains, and increase to 270 

 grains in the infant of one year; 

 but, with the development and exercise of the muscular system, it 

 wastes away, and may be reduced at twenty-one years of age to 

 a remnant weighing only forty grains. After twenty-five it is 

 rare, or difficult, to discover any of its structure left in the areolar 



1 xii. torn, viii, p. 675. 



2 en', p. 44. Peters confirms this, in ccxiu". p. 95, Taf. 4, fig. 5, gl. 



Thymus and heart of child at birth, ccxvi' 



