503 



derings throughout the body are much more easily followed than when 

 the movements of the normal corpuscles are observed. I have repeat- 

 edly injected milk, carmine, and aniline blue into the lymphatic spaces 

 of frogs, and found the substance injected in the course of a few hours 

 taken up in great quantity by the blood corpuscles. Now, it is by this 

 power of changing shape and performing spontaneous movements that 

 the white corpuscles appear to be able to leave the vessels. The veins 

 and arteries are formed of coats, all of which, except the internal, are 

 composed mainly of connective tissue ; for, as Cohnheim observes, even 

 the muscular coat may be considered as formed of connective tissue, 

 containing set in it a greater or less abundance of muscular fibre cells. 

 'Now, we know that the connective tissues, with the exception of carti- 

 lage, are full of spaces through which the corpuscles possessed of amse- 

 boid powers of locomotion can freely pass. But the internal coat of the 

 larger vessels is composed of a layer of flat epithelial cells united at 

 their edges, and a similar layer forms the only tunic of the smaller ca- 

 pillaries. Now, if this were a perfectly continuous layer, it would be 

 difficult to explain the passage through it of the white cells; for we 

 have no reason to believe that these bodies have any power of making 

 a way for themselves, but only of travelling through passages already 

 formed. But this difficulty is removed by a recent anatomical dis- 

 covery which shows that the internal vascular tunic is not absolutely 

 continuous, but that between the cells small circular spaces exist, called 

 stomata, numerous in the veins and capillaries, and more sparingly pre- 

 sent in the arteries, and which are readily seen after an injection of the 

 vessels with nitrate of silver, a reagent which brings out with great 

 distinctness the outlines of the epithelial cells. Through these stomata 

 Cohnheim, apparently with much reason, supposes the white corpuscles 

 to pass. 



"With regard to the emigration of the red corpuscles through the 

 capillary walls much difficulty exists. The red discs are generally be- 

 lieved to have no power of spontaneous movement, and Cohnheim sup- 

 poses that they are forced out mechanically through the stomata, 

 enlarged by the previous passage through them of the white cells, the 

 increased pressure in the capillaries being brought about by the partial 

 stasis of blood in the veins. The red corpuscles pass certainly in great 

 numbers through the capillaries of a part in which the venous circula- 

 tion is impeded, as may be seen in the web of the frog's foot after 

 ligature of the crural vein; but the pressure must be exerted in the 

 direction of the axis of the vessel, and it is difficult to conceive how 

 this could act on the corpuscles so as to force them to move through 

 the walls of the vessel in a direction at right angles to that of the force 

 acting on them. Professor Bastian, from these and some other consi- 

 derations, prefers to attribute to the red corpuscles powers of sponta- 

 neous movement, and to believe that the red as well as the white 

 corpuscles leave the vessels through powers inherent in themselves, and 

 independently of mechanical pressure. I have never myself seen any 



E. I. A, PEOC. VOL. X. 3 X 



