THYMUS AND THYROID GLANDS 



Thymus 



This large gland is found on the dorsal side of the head 

 above and mesial to the mandibular articulation. It is 

 located in the angle formed by the posterior part of the 

 masseter muscle and the mesial side of the ceratohyoid 

 externus. A slight removal of the fascia and superficial 

 blood vessels may be necessary to expose it fully. 



It is reddish gray in color and quite different from the 

 surrounding muscle tissues in this respect. When well 

 developed, it measures about 7 mm. long and 5 mm. wide. 

 It extends ventrally between the muscles for some distance. 



Make a thin slice with a razor blade and observe the 

 characteristics of the gland. If studied with a magnifica- 

 tion of X 25, numerous small cells, together with occa- 

 sional oval, nucleated blood cells will be seen. The small 

 cells are leucocytes. It is desirable to have prepared slides 

 for the observation of the microscopic structure. Compare 

 its structure with a similarly prepared slide of the thyroid. 



Thyroid 



The thyroid is on the ventral side of the branchial arches 

 in Necturus. To see it in position, spread apart the 

 ceratohyoid externus and the sternohyoid muscles until 

 the distal end of the first oeratobranchial can be seen 

 clearly. The gland is a small, thin, follicular disc that lies 

 on the distal end of the first oeratobranchial, spreads over 

 the proximal end of the first epibranchial and covers the 

 small second ceratobranchial. The gland rests on a sur- 



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