Billroth and others have also described a _spontaneous_ or _angioneurotic gangrene_ of the extremities, occurring during youth, in abrupt distinction to senile gangrene, whose course is tedious and painful, which will usually necessitate amputation. The cause of this condition has been found to be a well-marked arteriosclerosis and thrombosis, both in the arteries and veins. This form of gangrene occurs most often in the frigid zone--_e. g._, in Northern Russia. There are also forms of _visceral gangrene_, traumatic and non-traumatic, which often constitute fatal maladies. The latter are mainly due to thrombotic or embolic lesions, for example, the gangrene of the mesentery, already alluded to when discussing thrombosis (_q. v._), clinically described under Surgical Diseases of the Mesentery. =Gross Appearances.=--In a general way tissue death, known as _gangrene_, assumes two opposite types--_the moist_ and _the dry_. In _moist gangrene_, aside from those appearances which indicate commencing putrefaction of tissues, and the loss of heat due to stoppage of the blood supply, one of the most characteristic features is the formation of a so-called _line of demarcation_, _i. e._, a line which separates the dead from the living tissues. While this is usually plainly indicated by a red line which abruptly separates the discolored, usually dark, dead portion from the bright red, congested appearance of the living tissues, it is noted that this area of redness shades out into a more and more natural appearance as we pass upward, while below the line is seen a surface, usually covered with blisters, from which exudes a foul-smelling, altered serum, while the gangrenous portion assumes a dark, finally an almost black appearance, retaining only the crude outlines of its original shape. Along with this the objective evidences of putrefaction are unmistakable, appearances and odor being characteristic. With all there are more or less constitutional disturbances, and a recognizable, often a profound, condition of septic infection, due to the fact that along the line of demarcation absorbents are still active and that the poisonous products of putrefaction are being absorbed into the general system. Consequently _collapse_, _profuse perspiration_, _septic diarrhea_, etc., are noted. In gangrene from frostbite the process is slower than in the traumatic forms. In _gangrene from extravasation of urine_ the separation of sloughs is extensive, and sloughing of the scrotum with exposure of the testicles is a frequent result. In _decubitus_, or _bed-sore_, the process is still more slow, but always of the moist type. After a variable length of time there is separation of slough and a resulting large, often foul, ulcer.