TISSUES OF THE BODY. 1 33 



isolated examples of such fully formed, yet extremely delicate ceils may 

 be observed among the spherical and nucleated of a still earlier stage 

 of development. Thus the embryos of the rabbits which I have examined, 

 of about 9 mm. long, showed th to ^th of the whole number already with- 

 out a nucleus, and presenting the characteristic form of the blood-cell. 

 Kolliker found in sheep embryos of 8 -6 mm. long no such mature blood- 

 corpuscles, and Paget missed them also completely in a human foetus 9 mm. 

 in length. According to the first of these investigators, they are still 

 uncommonly rare in foetal sheep of 20 mm., while they constitute by far 

 the largest proportion of the cells in the young of the same animal measur- 

 ing 29 mm. In human embryos of the third month they only amount to 

 about from th to ^th of the whole mass of the blood. Sheep embryos, 

 on the other hand, of 11 to 29 mm. show a fall in the number of nucleated 

 cells to a very small fraction. 



The continued multiplication of red cells, which goes on naturally after 

 the process of division has ceased, appears to be carried forward in the 

 foetus, as in the adult, by the lymphatic glands, the medulla of bones, and 

 by the spleen. Very early the characteristic lymph-corpuscles, derived 

 from the latter sources, may be seen among the other coloured cells. That 

 the liver takes part in the formation of blood, as has been supposed, 

 appears very doubtful. 



REMARKS. Much has been adduced within the last few years in regard to the exit 

 through the walls of vessels of the cellular elements an occurrence of the highest 

 significance in pathological and physiological questions, and especially in inflamma- 

 tion. 



2. Lymph, and Chyle. 

 82. 



While describing the foregoing tissue we mentioned that certain of the 

 constituents of the blood are constantly passing from the capillary vessels 

 into the surrounding tissues in the form of watery solutions. 



This escape of fluid is indispensable for the nutrition of the various 

 parts of the body, the organs as well as tissues, in that in these solu- 

 tions various alimentary materials are contained. Now the latter, we 

 know, are different in the several' tissues; they are specially adapted, for 

 instance, to the wants of bone, of the brain, of muscle, and so on. Then 

 the fluids of the tissues become gradually quite different as to chemical 

 composition by the loss of various materials of nutrition in particular 

 parts of the body. 



But to these fluids are also added the results of the interchange of 

 matter going on in the tissues; the products of their decomposition; and 

 these are also, as has already been remarked in the chapter on general 

 chemistry, again different in the several organs. Here, then, we have a 

 new source of variety in constitution of the several tissue-juices of the 

 body. 



Now for the carrying off of the latter, as far as they do not immedi- 

 ately return to the stream of the blood by processes of diffusion ; the body 

 is supplied with a special system of fine canals which communicate by 

 means of their main outlets (already long known) with the circulatory 

 apparatus. Their mode of commencement is only partially understood at 

 present. 



This system of canals is known as the lymphatic or absorbent system. 



