33 THE DUCTLESS GLANDS. [CH. xxm. 



From the thymus various substances may be extracted, many 

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

 hypoxanthine, adenine, and leucine. 



The main constituent of the cells is proteid, and especially 

 nucleo-proteid. Indeed the thymus is usxially 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). 



function. 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 tem- 

 perature 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. 



Removal 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. 



Structure. The gland is encased in a capsule of dense areolar 

 tissue. This sends in strong fibrous trabeculze, which enclose the 

 thyroid vesicles which are rounded or oblong irregular sacs, con- 

 sisting of a wall of thin hyaline membrane lined by a single layer 

 of short cylindrical or cubical cells. These vesicles are filled 

 with transparent colloid nucleo-albuminous material. The colloid 

 substance increases with age, and the cavities appear to coalesce. 

 In .the interstitial connective tissue is a round meshed capillary 

 plexus, and a large number of lymphatics. The nerves adhere 

 closely to the vessels. 



In the vesicles there are, in addition to the yellowish glassy 



