160 HAND-BOOK OF PHYSIOLOGY. 



ner. A frog is urarized, that is to say, paralysis is produced by inject- 

 ing under the skin a minute quantity of the poison called urari; and the 

 abdomen having been opened, a portion of small intestine is drawn out, 

 and its transparent mesentery spread out under a microscope. After a 

 variable time, occupied by dilatation, following contraction of the minute 

 vessels and accompanying quickening of the blood-stream, there ensues a 

 retardation of the current, and blood-corpuscles, both red and white, 

 begin to make their way through the capillaries and small veins. 



"Simultaneously with the retardation of the blood-stream, the leu- 

 cocytes, instead of loitering here and there at the edge of the axial cur- 

 rent, begin to crowd in numbers against the vascular wall. In this way 

 the vein becomes lined with a continuous pavement of these bodies, which 

 remain almost motionless, notwithstanding that the axial current sweeps 

 by them as continuously as before, though with abated velocity. Now is 

 the moment at which the eye must be fixed on the t outer contour of the 

 vessel, from which, here and there, minute, colorless, button-shaped ele- 

 vations spring, just as if they were produced by budding out of the wall 

 of the vessel itself. The buds increase gradually and slowly in size, until 

 each assumes the form of a hemispherical projection, of width correspond- 

 ing to that of the leucoc}^te. Eventually the hemisphere is converted into 

 a pear-shaped body, the small end of which is still attached to the surface 

 of the vein, while the round part projects freely. Gradually the little 

 mass of protoplasm removes itself further and further away, and, as it 

 does so, begins to shoot out delicate prongs of transparent protoplasm from 

 its surface, in nowise differing in their aspect from the slender thread by 

 which it is still moored to the vessel. Finally the thread is severed and 

 the process is complete." (Burdon Sanderson.) 



The process of diapedesis of the red corpuscles, which occurs under 

 circumstances of impeded venous circulation, and consequently in- 

 creased blood-pressure, resembles closely the migration of the leuco- 

 cytes, with the exception that they are squeezed through the wall of 

 the vessel, and do not, like them, work their way through by amoeboid 

 movement. 



Various explanations of these remarkable phenomena have been sug- 

 gested. Some believe that minute openings (stigmata or pseudo stomata) 

 between contiguous endothelial cells (p. 133) provide the means of escape 

 for the blood-corpuscles. But the chief share in the process is to be found 

 in the vital endowments with respect to mobility and contraction of the 

 parts concerned both of the corpuscles (Bastian) and the capillary wall 

 (Strieker). Burdon-Sanderson remarks, "the capillary is not a dead 

 conduit, but a tube of living protoplasm. There is no difficulty in un- 

 derstanding how the membrane may open to allow the escape of leucocytes, 

 and close again after they have passed out; for it is one of the most strik- 

 ing peculiarities of contractile substance that when two parts of the same 



