THE RED PARTICLES. 2/9 



shape in different animals (CXLIII), will put this matter out of 

 dispute. In birds, as the common fowl for instance, the par- 

 ticles of the blood are oblong like a plum-stone (vide Plate V, 

 fig. 3), and the central particles found in the lymphatic glands of 

 that bird are also oblong, corresponding in every respect with the 

 central particle found in the vesicle of the blood of that animal. 



SECT. 91. In chapter III it was observed that the structure 

 of the thymus gland is similar to that of the. lymphatic glands, 

 and that it secretes from the blood particles like those secreted 

 by the lymphatic glands ; in fine, in its office, that it is no more 

 than a large lymphatic gland. But why should the thymus be 

 large in the foetus, and as the animal increases in size, become 

 smaller and smaller until at length it quite disappears ? Or, in 

 other words, why does not the thymus, like the liver or pan- 

 creas, continue through life? The reason why the thymus is 

 larger during the early part of life, is, we conceive, that it may 

 act as an auxiliary to the lymphatic system, for the purpose of 

 forming more of the central particles of the blood than could 

 have been made by the lymphatic glands alone during that 

 time, when nature wants them most; for the human body 

 grows more, in proportion to its weight, from the second month 

 after conception to the end of the third year, than it does in 

 any future period of its existence of no longer duration; a 

 greater quantity of blood is therefore wanted and applied by 

 the constitution in the quick growth of the animal than is 

 ever afterwards applied to that purpose in the same time. 



SECT. 92. If the thymus gland were wanting in the young 

 animal, the lymphatic vessels and glands must have been made 

 considerably larger than they now are, or out of proportion to 

 the other parts of the body; otherwise the animal could not 

 have been duly nourished, and the purposes of nature must 

 have been defeated; but by the assistance of the thymus, a 

 sufficient quantity of the central particles, to be converted into 

 blood necessary for the growth of the animal, are formed, 

 and nature at the same time preserves a just proportion in the 



(CXLIII.) The similarity in the figure of the lymph-corpuscle in 

 different classes of animals, and the difference in shape between this 

 round corpuscle and the long oval nucleus of the blood-disc of birds, 

 is explained in Note cxxu, p. 253. 



