78 LECTCTEE III. 



that we have in all three structures to deal with real 

 tubes, filled with now more, now less, moveable contents. 

 But this notion, however well it may accord with a 

 superficial view of the matter, does not express the 

 whole truth, inasmuch as we cannot compare the con- 

 tents of the different tubes. 



The blood which is contained in the vessels, cannot, 

 at least at present, be regarded as analogous to the axis- 

 cylinder, or the medullary [white] substance of a nerve- 

 tube, or to the contractile substance of a primitive mus- 

 cular fasciculus (Muskelprimitivbiindel [muscular fibre]). 

 I must, indeed, here remark, that the original develop- 

 ment of all the structures which may be included in this 

 group is still a subject of great controversy, and that 

 the view maintaining the simply cellular structure of 

 most of these elements is by no means completely estab- 

 lished. This much, however, appears to be certain, that, 

 at any rate in foetal parts, the blood-corpuscles are just 

 as much cells as the individual constituents of the walls 

 of the vessels within which the blood flows ; and that the 

 vessel cannot be designated as a tube which invests the 

 blood-corpuscles, as the cell-membrane does its contents. 

 It is therefore necessary in the case of the vessels to 

 draw a line between their contents and proper walls, 

 and to repudiate the seeming resemblance between the 

 vessels, and the nerves and muscular fibres. Again, if 

 we wished to adopt the mode of origin of the several 

 tissues as the basis of our classification, we should, in 

 accordance with prevailing views, have to associate the 

 lymphatic glands also with the blood, and might be 

 rather reminded of a connection such as we have seen 

 to exist in the relations between the epidermis and the 

 rete mucosum. But here I must once more impress 

 upon you that the lymphatic glands are distinguished 

 from glands properly so called, not only by their not 



