304 M. Treviranus cm the Organic Structure 



the presence or absence of the elementary cylinders, their size, 

 appearance, and arrangement in the different tissues of ithe body, 

 tend to throw much light on their individual natures as well as 

 on their mutual relations. These matters, however important, 

 I am reluctantly obliged to pass over in silence, in order to leave 

 room to dwell particularly on his seventh chapter, which treats 

 of the capillary vessels, and the roots or origins of the lympha- 

 tics, t 



" It has been believed by some, that the capillaries consist 

 merely of passages percolating the cellular tissue, but destitute 

 of any proper tunics. The following questions naturally occur 

 concerning these vessels. Have the capillaries proper tunics ? 

 Is their diameter large enough to admit the entrance of the red 

 blood ? or if their trunks are sufficiently large, have they rami- 

 fications too small for this purpose ? What is their connexion, 

 or have they any direct communication with the vessels carrying 

 red blood on the one hand, and the lymphatics on the other ? 



" One may easily distinguish with a strong magnifier that 

 these vessels, which are large enough to admit a stream com- 

 posed of several rows of globules, are marked with transverse 

 as well as longitudinal lines externally. It is not always easy to 

 determine whether the transverse lines are not mere folds or 

 plaits ; the longitudinal are formed by the outlines of the ele- 

 mentary cylinders of the cellular membrane which surrounds the 

 tunics of the vessels. The transverse lines vanish when the ves- 

 sel becomes too narrow to admit more than one row of the glo- 

 bules of the blood. Many vessels lose at this stage their cellu- 

 lar envelop, and retain only a single homogeneous tunic ; others, 

 on the contrary, retain a cellular coat. The simple vessels with 

 but one tunic occur chiefly in membranes that consist entirely of 

 a horny material, as, for instance, the epithelium which covers 

 the free side of the corpus ciliare, the ciliary processes and cili- 

 ary ligament as far as the edge of the cornea. Similar vessels 

 may be detected on the posterior surface and lateral edges of the 

 capsule of the lens, and in the pecten of birds. In these situa- 

 tions they sometimes occur of considerable dimensions. Thus in 

 the fox, this epithelium exhibits simple vessels with a diameter 

 of from 0,04 to 0,05 (millimetre). In the amphibia vessels of 

 this description may be detected in many other membranes. 



