798 PHYSIOLOGY OF DIGESTION AND SECRETION. 



ditions that threaten congestion of the brain is effected reflexly by means 

 of the hypophysis cerebri and the vagi. For details of this mechanism and 

 also of the supposed effect of the thyroid secretion on the irritability of the 

 centers innervating the heart and blood-vessels see " Archives de physiologic, " 

 1898, p. 618. 



Thy mus. The physiology of the thymus gland is very obscure, 

 indeed, practically nothing is known about its functions. Its prox- 

 imity to the thyroids and parathyroids and its general similarity 

 in origin would indicate that like them it may have some impor- 

 tant specific influence upon metabolism, but physiological experi- 

 ments so far have failed to discover what this influence is. Ac- 

 cording to Verdun the thymus arises from the endothelial pouches 

 belonging to the branchial clefts, chiefly from that of the third 

 cleft. Formerly it was supposed to reach its maximal develop- 

 ment at birth and subsequently to atrophy, being replaced by a 

 growth of lymphoid and fatty tissues. More recently doubt has 

 been thrown upon this belief. Several observers have stated that 

 it continues to increase in size after birth until the appearance of 

 puberty, and that true thymus tissue may persist throughout life. 

 Stohr, in fact, insists that what has usually been taken as lymphoid 

 tissue in the adult thymus is, in reality, epithelial or endothelial 

 tissue which presumably has some specific function. On the 

 physiological side Abelous and Billard have stated that extirpation 

 of these glands in the frog is followed by the death of the animal, 

 but later observers have failed to confirm this result either upon 

 frogs or mammals (guinea-pigs), so that we must believe that 

 whatever their function may be it is not absolutely essential to 

 the life of the organism. Injections of extract of the gland 

 (Svehla) cause a fall of blood-pressure and some quickening of the 

 heart-beat, but these effects are not specific. Unlike the thyroid 

 and parathyroid glands, the thymus contains no iodin (Mendel). 

 The only definite suggestion made regarding its influence is that 

 there is some sort of reciprocal relationship between it and the re- 

 productive glands. Castration (Henderson) causes a persistent 

 growth and retarded atrophy of the thymus, while removal of the 

 thymus (Paton) hastens the development of the testes.* 



Adrenal Bodies. The adrenal bodies or, as they are frequently 

 called in human anatomy, the suprarenal capsules belong to the 

 group of ductless glands. It was shown first by Brown-Sequard 

 (1856) that removal of these bodies is followed rapidly by death. 

 This result has been confirmed by many experimenters, and so far 



* References: Friedleben, "Die Physiologic der Thymusdriise," 1858; 

 Verdun, " Derives branchiaux chez les vertebras," 1898; Hammar, " Pfliiger's 

 Archiv," ex., and " Anatom. Anzeiger," xxvii.; Henderson, "Journal of Phys- 

 iology," 1904, xxxi., 222; Stohr, " Beit. z. Anat. u. Entwick," Anatom. 

 Hefte, 1906, xxxi., 409. 



