MR. SIMON ON THE COMPARATIVE ANATOMY OF THE THYROID GLAND. 297 
The position of the thyroid in this order of Reptiles is very definite and uniform ; 
it lies in the median plane of the body, just above the base of the heart, between the 
right and left carotid arteries. The thymus, for which it has been mistaken, lies 
away from the median line, along the outer side of the carotid on each side, between 
it and the subclavian artery. 
B. Emydosauria. 
In this order the thyroid gland is a single rounded organ, situate in the median 
plane of the body immediately above the base of the heart. A careful dissection in this 
region will first bring into view the thymus gland, which reaches on each side from 
just below the skull to within the thorax, and conceals the thyroid by the expansion 
of its pericardiac lobes in front of the base of the heart ; these being drawn aside the 
thyroid gland will be distinctly seen lying between the two carotid arteries. Carus* 
notices the elongated thymus of the Crocodile, and proposes the vague opinion that it 
may perhaps fulfil the double function of thymus and thyroid. 
C. Amphisbcenia. 
The thyroid gland in the Amphisbsenians occupies exactly the same site and rela- 
tions as in the true Serpents. 
D. Sauria. 
In this order it is not quite so easy as in the preceding ones to verify the existence 
of the gland, for it occupies different positions in different families, and is therefore 
very likely to elude superficial search : in some it is single and mesial, in others 
double ; in some it lies high in the neck, in others low. 
1. Among the Lacertidce, the arrangement of the gland is not quite constant for 
the family. In the true Lizards, and in the Teius, the gland is single, and of con- 
siderable transverse diameter, being so situated as to overbridge the trachea a little 
above the chest : in the Monitors, on the contrary, it is double ; one portion is placed 
on each side of the neck, in front of the carotid artery, and is connected with the in- 
ferior extremity of the corresponding half of the thymus gland. 
2. Among the Iguanidce likewise the anatomical relations of the gland are not quite 
uniform : in the Agamce and Iguarwe it is single, and lies transversely across the 
trachea ; while in the Istiurus, as in the Monitors, it consists of two portions, one 
lying in front of each carotid low in the neck. 
3. 4, 5. Among the Geckotidce, Chamoeleonidos and Scincidce, the same general form 
prevails as in the true Lizards ; the gland is single, and elongated transversely so as 
to reach across the trachea. In the Chamseleon it is rather higher (nearer to the os 
hyoides) than in the other families, and is overhung by the sacciform dilatation of the 
larynx. Treviranus'| v , in his dissection of the Chamceleo carinatus, noticed this body, 
and suggests of it that it “ may be compared to a thyroid gland.” 
* Op. cit. § 740. 
f Beobachtungen aus der Zootomie, p. 87. 
2 Q 2 
