THE CIRCULATION OF THE BLOOD. 155 



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

 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 outer contour 

 of the vessel, from which, here and there, minute, colorless, button- 

 shaped elevations 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 corresponding to that of the leucocyte. Eventually the hemi- 

 sphere 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 farther 

 and farther 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 increased 

 blood-pressure, resembles closely the migration of the leucocytes, 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. 



Yarious explanations of these remarkable phenomena have been sug- 

 gested. Some believe that the pseudo-stomata between contiguous endo- 

 thelial cells (p. 22) 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-San- 

 derson remarks: " The capillary is not a dead conduit, but a tube of living 

 protoplasm. There is no difficulty in understanding 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 striking peculiarities of con- 

 cractile substance that when two parts of the same mass are separated, 



