48 Dynamic Theory. 



ance persisting for a time in each individual, and then succumbing to 



adverse competition. 



The thyroid gland is an organ, the use of which is a matter of conject- 

 ure, and in adult life it is commonly regarded as a rudiment. It lies 

 in the front part of the throat, and is the part affected in the disease 

 called goitre. It may be of some sort of use in embryo life, probably 

 in influencing some of the secretions, but it is relatively smaller in the 

 adult, and although found in the ascidian and among the vertebrates up 

 to man, it is regarded by many as of no use, though perpetuated through 

 habit. It is not a true gland since it has no excretory duct, but it has 

 some sort of life since it has arteries and veins. 



Perhaps it is a parasite, like a sort of hereditary tumor reduced to a 

 definite habit of life and limit of growth. But is not every rudimentary 

 organ a parasite? 



The thyroid gland consists of numerous entirely closed sacs, 0. 04 to 

 0.1 mm. in diameter, which are embedded in a connective tissue consist- 

 ing of fibres and cells. The sacs contain a transparent, viscid, album- 

 inous fluid, and sometimes also colored 

 blood corpuscles. In the } T oung animal 

 the sacs are lined with a layer of cells. 

 In the surrounding tissue there are nu- 

 merous blood vessels and lymphatics. At 

 an early period the sacs dilate their cell- 

 ular lining and their contents undergo 

 colloid degeneration. 



The disease goitre consists of an ex- 

 tensive enlargement of the gland vesi- 

 cles. This disease sometimes produces 

 cretinism and so does the extirpation of 

 the gland. This gland in the tunicates, 

 is represented by a groove which secretes 

 a digestive fluid. In vertebrates it is 

 regarded by some as a blood corpuscle 

 forming gland, and as a regulator of the 

 formation of mucous in the body. What- 

 ever its function in early life, it loses or 

 totally changes it as age advances. 



Another singular rudimentary organ 

 in man and ape is the (ijfjtriulix rrnni- 

 formis or worm-like appendage to the cce- 

 cum or blind intestine. It is the si/u 

 of a goose quill and from ftiree to six 

 inches long, In some of the plant-eat- 



FIG. 67. Human stomach and intes- 

 tines. 



.1 Stomach. 

 2. Duodenum. 

 3. Jejunum. 

 4. End of the Hewn. 



B.C&cum. 

 Q. Ve 



rrmform appendix. ^JgSb 

 7. Axcemlmy colon. 

 8. Transverse " 



y.J)cxrrHtli)i.u " 

 10. Xifjmoid flexure of the colon. 

 11. Rectum. 

 12. Positioo of Kpleen. 



(Parts connecting 1 spleen with duo- 

 Uenuin, &c. are left off.) 



