DIAPEDESIS. 189 



immense importance of a due relation subsisting between the specific 

 gravity of the blood-plasma and that of the corpuscles.] 



In the vessels first formed in the incubated egg, as well as in those of young 

 tadpoles, the movement of the blood from the heart occurs in jerks (Spallanzani, 

 1768). The velocity of the blood-stream is influenced by the diameter of the 

 vessels, which undergo periodic changes of calibre. This change occurs not only 

 in vessels provided with muscular fibres, but also in the capillaries, which vary in 

 diameter, owing to the contraction of the cells composing their walls (p. 125). 



The velocity of the blood is greater in the pulmonary than in the 

 systemic capillaries (Hales, 1727); hence, we must conclude that the 

 total sectional area of the pulmonary capillaries is less than that of all 

 the systemic capillaries. 



95. Passage of the Blood-Corpuscles out of tile 

 Vessels Diapedesis. 



Diapedesis. If the circulation be studied in the vessels of the mesentery, we 

 may observe colourless corpuscles passing out of the vessels in greater or less num- 

 bers (Fig. 85). The mere contact with the air suffices to excite slight inflammation. 

 At first, the colourless corpuscles in the plasma-space move more slowly ; several 

 accumulate near each other, and adhere to the walls soon they bore into the 

 wall, ultimately they pass quite through it, and may wander for a distance into 

 the peri- vascular tissues. It is doubtful whether they pass through the so-called 

 " stomata" which exist between the endothelial cells, or whether they simply pass 

 through the cement- substance between the endothelial cells (p. 122). This process is 

 called Diapedesis, and consists of several acts : (a.) The adhesion of lymph -cells or 

 colourless corpuscles to the inner surface of the vessel (after moving more slowly 

 along the wall up to this point). (&.) They send processes into and through the 

 vascular wall, (c.) The body of the cell is drawn after or follows the process, 

 whereby the corpuscle appears constricted in the centre (Fig. 85, c). (d.) The com- 

 plete passage of the corpuscle through the wall, and its farther motion in virtue of 

 its own amreboid movements. Hering observed that in large vessels with peri- 

 vascular lymph spaces, the corpuscles passed into these latter, hence cells are 

 found in lymph before it has passed through lymphatic glands. The cause of the 

 diapedesis is partly due to the independent locomotion of the corpuscles, and it is 

 partly a physical act, viz., a filtration of the colloid mass of the cell under the 

 force of the blood-pressure (Hering) in the latter respect depending upon the 

 intra-vascular pressure and the velocity of the blood-stream. Hering regards this 

 process, and even the passage of the coloured corpuscles through the vascular wall 

 as a normal process. The RED corpuscles pass out of the vessels when the venous 

 outflow is obstructed, which also causes the transudation of plasma through the 

 vascular wall. The plasma carries the coloured corpuscles along with it, and at 

 the moment of their passage through the wall they assume extraordinary shapes, 

 owing to the tension put upon them, regaining their shape as soon as they 

 pass out (Cohnheim). 



