DIAPEDESIS. 



139 



The amount of water in the blood is of importance ; when it is increased, the 

 circulation is facilitated and accelerated ( 62). 



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

 capillaries ; so that the total sectional area of the pulmonary capillaries is less than 

 that of all the systemic capillaries. 



95- 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 numbers (fig. 116). 

 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 perivascular 

 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 be- 

 tween the endothelial cells (p. 94). This pro- 

 cess 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), (b) They send processes into 

 and through the vascular wall, (c) The body 

 of the cell is drawn after or follows the processes, 

 whereby the corpuscle appears constricted in 

 the centre (fig. 116, c). (d) The complete 

 passage of the corpuscle through the wall, and 

 its farther motion in virtue of its own amoeboid 

 movements. Hering observed that in large 

 vessels with perivascular lymph-spaces, the bmal vessel of a fro g f mesentery showing dia- 

 corpuscles passed into the spaces, hence cells P edesis ' w > ^vascular walls ; a a Poiseuille s 

 are found in lymph before it has passed through s P ace '> [' r > * ed . corpuscles ; I I, colourless 



corpuscles adhering to the wall, and c, c, m 

 various stages of extrusion ; /, /, extruded 

 corpuscles. 



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 intravascular 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). This remarkable phenomenon 

 was described by Waller in 1846. It was re-described by Cohnheim, and according to him the 

 out-wandering is a sign of inflammation, and the colourless corpuscles which accumulate in the 

 tissues are to be regarded as true pus-corpuscles, which may undergo further increase by division. 

 Stasis. When a strong stimulus acts on a vascular part, hypersemic redness and swelling 

 occur. Microscopic observation shows, that the capillaries and the small vessels are dilated and 

 overfilled with blood-corpuscles ; in some cases, a temporary narrowing precedes the dilatation ; 

 simultaneously the velocity of the stream changes, rarely there is a temporary acceleration, 

 more frequently it becomes slower. If the action of the stimulus or irritant be continued, the 

 retardation becomes considerable, the stream moves in jerks, then follows a to-and-fro move- 

 ment of the blood-column a sign that stagnation has taken place in other vascular areas. At 

 last the blood-stream comes completely to a standstill stasis and the blood-vessels are plugged 

 with blood-corpuscles. Numerous colourless blood-corpuscles are found in the stationary blood. 

 Whilst these various processes are taking place, the colourless corpuscles more rarely the red 

 pass out of the vessels. Under favourable circumstances the stasis may disappear. The 

 swelling which occurs in the neighbourhood of inflamed parts is chiefly due to the exudation 

 of plasma into the surrounding tissues. 



96. MOVEMENT OF THE BLOOD IN THE VEINS. In the smallest 

 veins coming from the capillaries, the blood-stream is more rapid than in the 

 capillaries themselves, but less so than in the corresponding arteries. The stream 

 is uniform, and if no other conditions interfered with it, the venous stream 

 towards the heart ought to be uniform, but many circumstances affect the stream 



lymphatic glands. The cause of the diapedesis 

 is partly due to the independent locomotion of 

 the corpuscles, and it is partly a physical act, 



