70 CHANGES IN THE CIRCULATION OF THE BLOOD. 



endothelial cells. Although no marked morphological changes have 

 as yet been detected which explain this extravasation, it is probable 

 that some change in the nutrition of the walls does occur which renders 

 them more permeable. Haemorrhage by diapedesis is apt to occur as a 

 result of A^enous congestion, or when the flow of blood in the smaller 

 vessels has been suspended for some time; or it may result from the 

 action of some poison, or from an injury not leading to rupture ; or it 

 may occur in incompletely developed blood-vessels, in tumors and other 

 new -formed tissues. 



In the extravasation of blood by diapedesis the white blood cells may 

 pass through the walls of the vessels, partly at least in virtue of their 

 amoaboid movements ; the red cells, on the other hand, having no power 

 of spontaneous movement, are, according to Arnold, carried passively 

 through the walls by minute currents of fluid which, under the changed 

 condition, stream in increased force and volume through the endothelial 

 cement substance into the tissue spaces outside. 



The altered condition of the blood-vessels leading to haemorrhage may 

 be local or general, and in the latter case it may either be congenital, as 

 in some cases of the hseniorrhagic diathesis, or it may be the result of 

 a general disease, such as scurvy, purpura, etc. The presence of bacteria 

 in the vessels, or of bacterial or other poisons in the blood, as in malig- 

 nant endocarditis and in haemophilia ueouatorum, may induce changes in 

 the walls of the vessels, leading to extravasation. 



Very small haemorrhages are called petechice ; larger, diffuse accumu- 

 lations of blood in the interstices of the tissues are commonly called ec- 

 chymoses or suggillaiions. A complete infiltration of a circumscribed por- 

 tion of tissue with blood is called a hcemorrlwgic infarction. A collection 

 of blood in a tumor-like mass is called a hcematoma. Sometimes the ele- 

 ments of the tissue into which the blood escapes are simply crowded 

 apart ; sometimes, as in the brain, they are broken down. 



The extravasated blood in the tissues usually soon coagulates, al- 

 though exceptionally it remains fluid for a long time. A certain num- 

 ber of the white blood cells may wander into adjacent lymph vessels, or 

 they may remain entangled with the red cells in the meshes of the fibrin. 

 The fluid is usually soon absorbed ; the fibrin and a portion of the white 

 blood cells disintegrate and are absorbed. The red blood cells soon give 

 up their haemoglobin, which decomposes and may be carried away or be 

 deposited either in cells or in the intercellular substance at or near the 

 seat of the haemorrhage, either in the form of yellow or brown granules 

 or as crystals of haematoidin. Sometimes all trace of extravasations of 

 blood in the tissues disappears, but frequently their seat is indicated for a 

 long time by a greater or less amount of pigment or by new-formed con- 

 nective tissue. Occasionally the blood mass, in a more or less degenerated 

 condition, becomes encapsulated by connective tissue, forming a cyst. 



The action of phagocytes (see page 116) in the disposal of dead ma- 

 terial is here, as it is under a great variety of conditions, an important 

 factor in the restoration of the body after lesion to its normal conditions. 



