158 DISEASES OF THE PAKENCH1MA OF THE LCNG. 



peripheral infarctions preserve both the size and the cuneiform shape of 

 the superficial lobuli. Upon careful examination of an arterial branch 

 within whose range a hsemorrhagic infarction has formed, we find in it 

 a clot by -which its calibre is more or less obstructed. This is easily 

 demonstrated in the larger vessels, but in the very small ones it is some* 

 times difficult 



That the obstructing coagulum hag not formed at the place of its 

 lodgment, but that it comes from some remote region of the body, 

 whence it has become detached and swept into the current of the blood, 

 until, finally, it has become impacted in some branch of the pulmonary 

 artery too narrow to admit of its passage, has long been recognized 

 as the conditions under which haemorrhagic infarction arises. The 

 credit of this valuable discovery is due to Virchow. That investigator, 

 by introducing particles of fibrin, muscle, elder-pith, and the like into 

 the jugular veins of dogs, demonstrated by dissection that these foreign 

 bodies blocked up branches of the pulmonary artery, and produced 

 haemorrhagic infarctions, lobular pneumonia, and small abscesses, beyond 

 the points obstructed. Conversely, he proved by dissection of bodies, in 

 which the diseased spots so long known as metastases had been found, 

 that the arteries leading to the affected points were occluded by an 

 embolusa, fibrinous plug, which undoubtedly had proceeded from a 

 thrombosis of a superficial vein, or from particles whose origin was in- 

 disputably traceable to some region of suppurative or sanious liberation 

 upon the surface. 



Of late, the doctrines of pyaemia and of septicaemia have undergone 

 many revolutions ; but that of embolism that is, of the dependence of 

 haemorrhagic infarction upon the introduction of clots, or of particles of 

 tissue into the circulation has remained unshaken. 



It is easy to understand why metastatic infarctions of the lungs are 

 caused by emboli from disintegrating thromboses of peripheral veins, or 

 from suppurating or sanious surfaces. When an embolus is detached 

 from its point of origin by the current of the blood, it meets with no 

 obstacle on its way to the heart, as the veins through which it travels 

 are constantly growing larger. It passes unhindered into the right heart 

 and into the pulmonary artery, and is not arrested nor impacted until it 

 arrives at some branch of the latter whose diameter is less than its own. 

 Upon similar grounds, it is the rule for emboli, which originate from the 

 roots of the portal vein, or which enter the portal vein in cases of ulcera- 

 tion or of sanious discharge from the intestines, to pass into the ramifica- 

 tions of the portal vein within the liver, causing metastases in that 

 organ, and for emboli which come from the lungs or the left side of the 

 heart to occlude the arteries of the spleen, kidneys, or brain. Where 

 exceptions to this rule occur, as when we sometimes find infarctions in 



