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MANUAL OF HISTOLOGY. 



measurements vary greatly at different times in the same ani- 

 mal, and even more so in different animals. It may be stated, 

 in general, that the calibre corresponds to the size of the largest 

 blood-globules. In man, therefore, we have an average diam- 

 eter of about 0.007 mm. The largest capillaries exist in the 

 mucous membrane of the stomach and colon, the periosteum 

 and bones, and the pituitary body. The smallest are found in 

 the skin, the small intestine, the lungs, the muscles, the gray 

 substance of the brain, and the retina (Valentin, Weber, and 

 Henle). 



The genesis, reproduction, and regeneration of capillaries. 

 There is still much uncertainty about the mode in which 

 blood-vessels are first formed in the embryo. My personal 



FIG. 63. Growth and development of capillaries by nucleated sprouts of protoplasm : A, poly-nucle- 

 ated large sprout with filiform process ; B, B, blood-globules ; C, branched cell ; D, delicate protoplasmic 

 tendril linking C with E, a smaller mono-nucleated sprout of endothelial wall. 



observations on this subject, while working recently under the 

 supervision of Kolliker, appear to confirm the view held by 

 Foster and Balfour. These authors' account of the interesting 

 process may be summed up as follows : About the second day 

 of incubation in the chick, certain mesoblastic cells send out 

 solid processes, which, uniting, form a protoplasmic network 

 containing nuclei. A majority of the latter acquire a reddish 

 tint, and are ultimately transformed into colored blood-glob- 

 ules. Other nuclei, however, remain unaltered, and, receiving 

 an investment of protoplasm, form walls inclosing the reddened 



