VASCULOGENESIS IN THE CAT 13 



The positive conclusion that extravasation means communi- 

 cation between vessel and tissue space seems justified when the 

 injection is controlled by a manometer and practised at physio- 

 logical or subnormal pressure, as in Herring and Simpson's 14 in- 

 jections of the liver or Pustoiwoitow's 15 of the spleen. If, as in 

 these cases, the injected substances escape under less than normal 

 pressure it is permissable to infer a real continuity, though strictly 

 speaking it is necessary to confirm this by direct observation of 

 the tissue (vide infra). 



But if the pressure is not thus controlled, it will always be pos- 

 sible to interpret the extravasation as an artifact, for it can always 

 be argued that rupture occurred under excessive pressure. Cen- 

 trifugal injections of lymphatics are obviously subject to this 

 doubt, so that the presence here of extravasation is only suggest- 

 ive and not demonstrative of communication between these ter- 

 minals and the tissue spaces. 



Other methods of control than that of manometric measure- 

 ment of the pressure can not be freed from subjectivity. To 

 observe, for example, the passage of the injection with a binocu- 

 lar and discontinue when it begins to extra vasate is arbitrary; 

 to succeed in stopping at the moment when extravasation has 

 just begun, when the terminals have still but a mossy appear- 

 ance, shows only the point to which the observer's dexterity 

 may be developed. One may, with practice, heat a thermometer 

 rapidly till the mercury rises to a chosen point, and become able 

 to discontinue just as the point is reached. The repetition of 

 the act may increase exactitude, but will not prove the absence 

 of luminal continuity beyond, however we might argue, when 

 we had just passed our selected point, that we had ruptured a 

 delicate and invisible membrane. 



Nor is even the "explosive" (MacCallum) rupture of the vessel 

 wall at some intermediate portion of its course a proof that its 

 terminals are closed. A very moderate experience with the in- 

 jection of adult material will convince anyone that large veins 

 may rupture before their radicles are filled, and the rupture may 



14 1906, Proc. Roy. Soc. B., vol. 78. 



15 Arch. f. Anat. u. Entwickelungsgesch., p. 219. 



