304 STUDIES IN ADVANCED PHYSIOLOGY. 



longed the contractions of the voluntary muscles. The ef- 

 fects of such an injection are soon worn off, showing that 

 the active principle of the extract is soon destroyed or 

 eliminated from the body. It will be seen, however, that 

 at best our knowledge is in a most unsatisfactory state, and 

 while it is possible that these adrenals may be concerned in 

 forming some kind of tonic substance which proves bene- 

 ficial and indispensable to the proper working of other tis- 

 sues, it seems more probable that it is concerned with the 

 removal of injurious substances already present in the 

 blood. This latter view is materially strengthened by the 

 observation that Addison's disease, marked especially by 

 accumulation of such pigment, follows abnormal conditions 

 of these adrenal bodies. 



4. The Thymus Gland. The thymus gland is a tem- 

 porary structure which is quite large in early life, reaching 

 its maximum size about the second or third year of life, and 

 then gradually dwindling away until adult life, when it is 

 practically gone. It is situated in the lower region of the 

 neck and the upper part of the chest, and when at its max- 

 imum development is quite large. Its dimensions about 

 the time of birth are two inches in length and about one 

 and a half inches in width, with a thickness of about one- 



Fig. 124. PART OF THE MEDULLA OF A THYMUS GLAND, SHOWING SEVERAL OF THE 



RETICULAR FIBRES, A NUMBER OF LYMPHOID CELLS CALLED THYMUS CORPUSCLES, a; 



AND TWO CONCENTRIC CORPUSCLES, 6. (After Cadiat.) 



third of an inch. In some of the lower animals this gland 

 may reach rather remarkable dimensions. In a calf it may 

 be as much as six or eight inches in length and two or 



