740 SPECIAL PHYSIOLOGY. 



on the head, by which it is alleged that the muscles of the neck compress the 

 veins, and so cause congestion, and ultimate enlargement of the thyroid body. 

 In the canton of the Valais, where go tre prevails in its most intense form, 

 it is often associated with an arrest of development of the whole frame, espe- 

 cially of the skull and brain, which constitutes the condition known as Cretin- 

 ism. The Cretin may, indeed, be said to be a small idiotic human being, dis- 

 tinguished from ordinary idiots, by the thyroid body being enlarged or goitrous. 

 But in the Cretin districts, persons of full stature, of duly proportioned cranial 

 and cerebral development, and of ordinary intellectual capacity, are seen with 

 goitres larger even than those found in Cretins themselves. 



The thymus body, or thymus gland. This ductless gland is a tem- 

 porary organ in the animal economy. Present in the embryo, it attains 

 its largest relative size to the body in the infant, and seems to be most 

 active in function a short time after birth, growing up to that period 

 even faster than the body. It then continues to grow, so as to keep 

 pace with the rest of the body, up to the age of two years; but soon, 

 it no longer increases with the body, and, at about twelve years of age, 

 is usually changed into a fatty mass; according to Friedleben, it may 

 grow a little after the second year, and riot become fatty until after 

 puberty. Finally, especially in thin persons, it gradually wastes, so 

 as to leave nothing but a mere vestige behind. 



In its most complete condition, it forms a double organ, composed 

 of two lateral irregular lobes, joined by a central mass, and situated 

 partly in the lower region of the neck, and partly in the thorax, lying 

 upon the trachea and the great bloodvessels. It measures, at birth, 

 about 2 inches in length, and weighs half an ounce. It is a soft pink- 

 ish-gray body, consisting on each side of a string of compressed lobules, 

 connected together by an elongated part, like a cord. A strong areolar 

 coat incloses and connects the various lobules, and sends intervening 

 coverings between their ultimate subdivisions. The lobules, or acini, 

 are composed of a soft milk-white parenchyma, consisting of granular 

 matter, nuclei, and nucleated cells; the central part of each lobule is 

 so soft or fluid that, when opened, a cavity is found, which extends 

 into the secondary lobules, of which the primary ones are composed. 

 The cord which connects the lobules together contains the same paren- 

 chymatous substance, and is likewise soft or fluid in the centre, so as 

 to form a cavity, called the reservoir of the thymus ; this communicates 

 with the soft cavities of all the lobules, and also with certain small 

 sacculi situated in its walls. Each lateral half of the thymus has its 

 proper reservoir, the two sometimes communicating through the cen- 

 tral transverse mass. The cavities within the lobules and connecting 

 cord are not lined by a distinct limitary membrane and epithelium ; 

 the fluid within them is milky white, and resembles chyle. It con- 

 tains nuclei and nucleated cells, similar to those of the white paren- 

 chyma itself. Many of these closely resemble the developing lymph- 

 corpuscles found in the loculi of the lymphatic glands, and, therefore, 

 also the white corpuscles of the blood. No minute fatty molecules, 

 similar to those forming the " molecular basis" of the chyle, are found, 

 however, in the white fluid of the thymus. To chemical analysis, this 

 body yields about 20 per cent, of solid matter, chiefly albumen, some 

 gelatin, only a little fatty matter, and traces of sugar, leucin, sarcin, 



