THYMUS OF REPTILES. 565 



In the Frog they lie on the carotids, also close to the basi- 

 branchials, or thyrohyals. 



In a Python of ten feet in length the thyroid was an oval body, 

 ten lines by six lines in the two diameters, lodged in the fork 

 made by the divergence of the large and small carotids, and 

 having the smaller thymus bodies, one on each side. 



In the true Lizards (Lacerta) the thyroid is single, but broader 

 than it is long ; in the Monitor it is double : it is single in Geckos, 

 Skincs, and Chameleons, but has a more advanced position in the 

 latter, where it is underlapped, or covered, by the laryngeal pouch. 



In Chelonia the thyroid, as in Serpents, lies between the two 

 carotids, but is usually covered by the pericardial part of the 

 thymus. The constituent vesicles are from ^ to -^ inch diameter, 

 closely aggregated : the epithelial lining contains a row of nuclei 

 imbedded in the granular substance, fig. 

 278. Among the contents of the vesicles 377 



were found, in most, ' one to three yellow- 

 ish, coarsely granular globules, -g-~ to 



tttSttt inch diameter.' ' A fine large octo- P 



10 o Portion of the wall of a vesicle from 



hedral crystal was also seen in one of the the thyroid of a Tortoise, cclxxxix. 

 cavities.' 1 



§ 104. The Thymus Body or Gland of Reptiles. — The ductless 

 gland-like tubulo-vesicular body to which the name ' thymus ' 

 can, with any homological probability, be given makes its first 

 appearance in the Vertebrate series with the establishment of 

 lungs as the main or exclusive respiratory organ. 2 Thus in Siren 

 and Proteus the thymus is wanting, as in all Fishes: in tailed 

 Batrachia {Menopoma, Triton) it is represented by a pair of 

 bodies situated, one on each side, near the origin of the pulmonary 

 artery. They make their appearance near the same part, but 

 rather more in advance of the pericardium, in the larva? of 

 anourous Batrachia, and often degenerate into fat in the old Frog 

 or Toad. In most Ophidia the thymus lies on each side of the 

 carotid, of elongate form and unequal size : sometimes in two or 

 more distinct parts : usually associated with, and concealed by, 

 fatty matter. In a Python of ten feet in length I found the 

 thymus in two bodies, each about the size of a pea, of a yellowish 

 colour, one situated on the termination of the right jugular, the 

 other between the origin of the larger carotid and the left jugular. 

 Between the thymus bodies was the much larger single thyroid. 

 In the Psammosaur and Iguana the thymus is broad and flat, 

 covers sternally the thyroid, extends along the common trunk of 



1 CCLXXIX. ]>. 1100. 2 CXVIII. 



