X 



THE DUCTLESS GLANDS 



General. 



Apart from histological considerations, and regarded from a purely 

 anatomical point of view, there is not a great deal to be said relevant 

 to the ductless glands of Salamandra. They were first subjected to 

 a detailed investigation in 1888 by Maurer, who studied not only 

 their character and histology in the adult, but also their origin and 

 development. Bolau (1899) undertook a fresh investigation of the 

 histology of the thyroid and thymus of a large number of amphibian 

 types, including Salamandra, while Wilder (1929) discussed the signi- 

 ficance of the ultimo-branchial body in Urodeles. This practically 

 exhausts the anatomical literature of note, but papers dealing with 

 the physiology of the organs concerned are much more numerous. 



I. The Thyroid (Thyreoid) Gland (Figs. 37 and 38, gl.thy.). 



According to Maurer, the thyroid develops as an unpaired struc- 

 ture which very soon divides into two. It makes its appearance very 

 early in the developing embryo. The thyroid in the adult consists of 

 a pair of elongated ovoid glands about ^'S ^^- long by i mm. broad, 

 lying on the floor of the throat at about the level of the os triangulare, 

 immediately in front of the arterial arches, lateral to the MM. genio- 

 hyoideus and rectus cervicis superficialis, and posterior to the M. 

 interhyoideus. Each gland is enclosed in a tough connective tissue 

 capsule into which a few fibres from the M. genio-hyoideus are in- 

 serted. The glands are nourished by the thyroid arteries, branches 

 of the external carotids, the blood being returned to the heart by the 

 thyroid veins (p. 226). The hypoglossal nerve crosses the ventral 

 aspect of each gland. Even with the naked eye, or at any rate with 

 a hand lens, each gland is seen to consist of about 20 translucent 

 follicles. The microscopic examination of transverse sections shows 

 that each follicle consists of a single layer of cells which stain very 

 deeply with iron-haematoxylin, and is filled with a clear colloidal 

 fluid, the exact size and appearance of the follicles depending of 

 course on the seasonal and physiological conditions prevailing at 

 the time of fixation. 



As is well known, the secretion from the thyroid has a very 



