84 ELEMENTS OF HISTOLOGY. [Chap. xi. 



acquiring more and more of a tubular form. In the 

 case of an isolated cell, its protoplasmic processes 

 grow by degrees to the nearest capillary, to the wall 

 of which they become fixed, and the cavity of the cell 

 finally opens through such processes into that of the 

 capillary vessel. 



The wall of young capillaries is granular-looking 

 protoplasm (the original cell substance), and in it 

 are disposed, in more or less regular fashion, oblong 

 nuclei, derived by multiplication from the nucleus of 

 .the original cell. In a later stage, a differentiation 

 takes place in the protoplasmic wall of the capillary 

 into cell-plates and cement substance, in such a way 

 that each of the above nuclei appertains to one cell- 

 plate, which now represents the final stage in the 

 formation of the capillary. Both in the embryo and 

 in the adult a few isolated nucleated protoplasmic cells, 

 or a few protoplasmic solid processes of an existing 

 capillary, may by active and continued growth give 

 origin to a whole set of new capillaries (Strieker, 

 Affanasieff, Arnold, Klein, Balfour, Ranvier, Leboucq). 



CHAPTER XI. 



THE LYMPHATIC VESSELS. 



110. THE large lymphatic trunks, such as the 

 thoracic duct, and the lymphatic vessels passing to and 

 from the lymphatic glands, are thin- walled vessels, 

 similar in structure to arteries. Their lining endothe- 

 lium is of the same character as in an artery, and so are 

 the elastic intima and the media with its circular mus- 

 cular tissue, only they are very much thinner than in 

 an artery of the same calibre. The adventitia is an 



