NECflOSIS OF TISSUES. 5 



as to give the part a dark purple or livid hue. In such a case, the 

 microscope shows us an extraordinary turgidity of the capillary vessels, 

 associated with minute extravasations scattered throughout the paren- 

 chyma ; in the smaller veins, which are also gorged with blood, we find 

 blood corpuscles, solitary or in rows, intercalated between the layers 

 of which their walls consist. On inquiring into the cause of such a 

 disturbance of the circulation, we commonly find that the obstacle is 

 situated in the aSerent arteries of the part. In a future page, those 

 diseases of the vessels will have to be specially considered, which may 

 give rise either to simple plugging (thrombosis, embolism) or to gradual 

 contraction of the calibre of a vessel, or finally to a condition of its walls 

 which so exhausts the vis a tergo of the heart in the larger arterial 

 trunks that it sinks to a mere nothing in the extremities. {Special Parf, 

 Chap. II.). Enfeeblement of the heart's contractions, consequent upon 

 disease of its muscular substance, or due to general debility, such as 

 follows typhus fever, is also capable of disturbing the circulation in the 

 peripheral parts of the body to such an extent as to cause mortification. 

 Senile gangrene, which affects the toes, feet, and legs as far up as the 

 knees, is commonly caused by the co-operation of both causes, i.e. by 

 changes taking jDlace in the muscular walls of the heart, together with 

 disease of the vessels. Compression of the arteries {e.g. by tumours 

 from without) must naturally exert a like unfavourable influence upon 

 the circulation within the area of their distribution. Tonic spasm of 

 its muscular coat very rarely contracts the tube of an afferent artery so 

 far as to check the flow of blood through it. Yet gangrene of the 

 extremities, following the consumption of large quantities of ergotized 

 grain, is believed to be due to this cause. 



The stoppage of the blood current may also occur in the capillaries 

 themselves. The most interesting example of this is when the capillaries 

 of a part are compressed by exudation or morbid growth in its paren- 

 chyma. So in diphtheritic inflammation, an exudation into the substance 

 of a mucous membrane, or of the skin, causes death and sloughing of 

 the part. Moreover, most of the necroses (so-called KaTk^o^qv) of the 

 osseous system come under this head; collections of pus jjetween the 

 periosteum and the bone, due to periostitis, compress the nutrient 

 arteries which pass from the former to the latter, and so deprive 

 the outermost layers of the bone of their supply of pabulum. The 

 formation of pus in the Haversian canals will lead in like manner 

 to squeezing of vessels and death of corresponding portions of bone 

 tissue (caries). It need hardly be added that in these cases no con- 

 gestion of the necrosed part can possiblj^ occur; on the contrary, we 

 may invariably expect to find a marked degree of anaemia. An obstacle 

 to the escape of blood through the veins rarely causes gangrene. For, 

 to produce gangrene, it seems that the arrest must be complete; and 

 this condition is hardly ever realised. At least we often see thrombosis 

 of all the greater veins of the thigh after pregnancy, without its being 

 followed by gangrene of the leg. Indeed, there is only one case whicli 

 comes under this head, viz. when a part is jammed into a dispropor- 



