On the comparative anatomy of the thyroid gland / [John Simon].

  • John Simon
Date:
1844
Catalogue details

Licence: Public Domain Mark

Credit: On the comparative anatomy of the thyroid gland / [John Simon]. Source: Wellcome Collection.

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    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. 13. 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. Amphisbasnia. The thyroid gland in the Amphisbeenians 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 Iguanas 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, Chamasleonidce and Scincidas, 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^, 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. t Beobachtungen aus der Zootomie, p. 87. 2 Q 2
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