CH. XXIII.] THE THYMUS AND THYROID 335 



consist of a nucleated granular centre, surrounded by flattened 

 nucleated epithelial cells. These are islands of epithelial cells cut off 

 from the epithelium of the pharynx in process of development. They 

 are not occluded blood-vessels, as was at one time supposed. They 

 remind one somewhat of the epithelial nests seen in some varieties of 

 cancer. 



The arteries radiate from the centre of the gland. Lymph sinuses 

 may be seen occasionally surrounding the periphery of the follicles 

 (Klein). The nerves are very minute. 



From the thymus various substances may be extracted, many of 

 them similar to those obtained from the spleen, e.g., xanthine, hypo- 

 xanthine, adenine, and leucine. 



The main constituent of the cells is proteid, and especially nucleo- 

 proteid. Indeed, the thymus is usually employed as the source of 

 nucleo-proteid when one wishes to inject that substance into the 

 blood-vessels of an animal to produce experimentally intravascular 

 clotting. It is, however, not characteristic of the thymus, but is 

 found in all protoplasm. The method of preparation will be given 

 later (see Coagulation of Blood). 



The thymus takes part in producing the colourless corpuscles like 

 other varieties of lymphoid tissue. In hibernating animals it exists 

 throughout life, and as each successive period of hibernation approaches 

 it greatly enlarges and becomes laden with fat. Hence it appears to 

 serve for the storing-up of materials which, being reabsorbed during 

 the inactivity of the hibernating period, may maintain the respiration 

 and the temperature of the body in the reduced state to which they 

 fall during that time. Some observers state that it is also a source 

 of the red blood-corpuscles, at any rate in early life. 



Eemoval of the thymus in the frog (in which animal it persists 

 throughout life) produces muscular weakness, paralysis, and finally 

 death. Intravenous injection of extracts of thymus lowers blood- 

 pressure, though the heart may be somewhat accelerated. 



The Thyroid. 



The thyroid gland is situated in the neck. It consists of two 

 lobes, one on each side of the trachea, extending upwards to the 

 thyroid cartilage, covering its inferior cornu and part of its body; 

 these lobes are connected across the middle line by a middle lobe 

 or isthmus. It is highly vascular, and varies in size in different 

 individuals. 



The gland is encased in a capsule of dense areolar tissue. This 

 sends in strong fibrous trabeculae, which enclose the thyroid vesicles 

 which are rounded or oblong irregular sacs, consisting of a wall of 

 thin hyaline membrane lined by a single layer of short cylindrical 



