THE THYMUS GLAND 113 



The normal weight of the thymus in man is shown by the 

 following table (Hammar) : 



At birth ... ... ... ... 13.26 grammes. 



Between the ages 1-5 ... ... 22.98 ,, 



,, ,, ,, 6-10 ... 26.1 ,, 



M 11-15 ...... 37-5 2 



, 1 6-20 ...... 25.58 



, ,, 66-75 ...... 6.0 ,, 



At a more advanced age the organ is principally composed 

 of adipose tissue. 



The size of the thymus in the dog is estimated by Baum as 

 follows : At birth, the weight of the thymus is to the body as 

 i 1250. In the first two weeks the proportion changes to i :ijo. 

 Involution now begins, and in the course of the first two to three 

 months, the proportion falls to i :i2OO, or i :i6oo. Remnants 

 of the thymus are found in dogs up to the second or third year. 

 Basch found that the most frequent proportional figures were : 

 in dogs in the third to fourth week, i : 300; and in rabbits in the 

 third week, i 1500 or 600. According to Basch, the development 

 of the thymus in dogs, during the same period, is at least twice 

 as favourable as in rabbits and cats, and for this reason, dogs 

 are more suitable subjects for experimental extirpation. 



According to Noel Paton and A. Goodall, guinea-pigs are 

 the best subjects for this purpose. In these animals the thymus 

 appears in the form of two flat lobes ; it is situated in the neck 

 upon both sides of the trachea and does not extend into the 

 thoracic cavity. In new-born animals, its average weight is 

 0.19 grm. to about 80 grm. of body weight, and the weight of the 

 organ continues to increase until the weight of the animal is 

 about 300 grm., which occupies about two months. At this age 

 the animal becomes sexually mature and retrogressive changes in 

 the thymus begin to take place; in animals weighing 400 grm. 

 and over, its average weight is only 0.03 grm. 



According to the investigations of Soederlund and Backmann, 

 the thymus of rabbits attains its maximum relative weight at the 

 end of the third week after birth ; and its greatest absolute weight 

 during the fourth month, while the organism is preparing for 

 spermatogenesis. The involution of the thymus seems to coin- 

 cide, in rabbits as in guinea-pigs, with the completion of the 

 development of the sexual organs. 



PHYSIOLOGY OF THE THYMUS. 



In 1858, a monograph by Friedleben appeared, entitled " The 

 Physiology of the Thymus Gland in Health and in Disease." 

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