Febkuaby 8, 1907] 



SCIENCE 



205 



the aneurisms frequent at this point, as 

 they obstruct the vessels and raise the ar- 

 terial tension behind the points where they 

 have formed a blockade. This induces in 

 a purely mechanical way a distension of 

 the vessels at the region of increased pres- 

 sure. 



Even much more complicated changes in 

 the host may be reduced in final analysis 

 to a mechanical effect. In the case of the 

 Egyptian blood fluke, the female goes into 

 the venous plexus of the pelvis to oviposit 

 and the oval eggs which are supplied with 

 a rudimentary spine at one end are carried 

 into the capillaries by the blood current 

 and there gradually work their way 

 through the wall of the alimentary canal, 

 or the urinary bladder, so as to reach the 

 cavity of the organ and to escape from the 

 body. The large numbers of these eggs 

 which are produced fill up the capillaries, 

 interfere with the flow of blood, lacerate 

 the capillary walls and tissues through 

 which they are forced mechanically until 

 a series of serious changes is invoked in the 

 organs in question. The condition is even 

 more serious when these same eggs chance 

 to be carried about the body of the host by 

 the circulating blood, become entangled in 

 the capillary network at different points 

 and constitute the foci of small emboli. 

 Such may arise in the brain where retarda- 

 tion of the blood current and the resulting 

 emboli are sources of serious danger to the 

 host, since they give rise to brain tumors 

 and may evoke apoplectic symptoms. The 

 effects produced by such brain tumors of 

 parasitic origin do not differ from those of 

 other tumors or other foreign bodies. All 

 of these effects are at basis mechanical and 

 the same results would apparently be pro- 

 duced by any inert bodies of similar form 

 and number. 



One may go still further and call atten- 

 tion to the fact that in some eases it is a 

 mass of embryos which constitute the 



mechanical influence at the basis of serious 

 changes. Some of the filarise which inhabit 

 the connective tissue are viviparous and 

 produce countless numbers of embryos; 

 these are carried by the lymph and blood 

 stream all over the body; accumulating, 

 evidently by chance, in considerable num- 

 bers at certain points of the lymph vessels, 

 for instance, they act mechanically to pro- 

 duce lymph stasis and dilation of the parts. 

 The long-continued working of this cause 

 will produce an enormous distension of 

 some regions of the body, giving rise to 

 one form of the condition which is known 

 as elephantiasis. 



Another mechanical influence of the 

 passive parasite is traceable to the pressure 

 which it exerts upon surrounding tissues. 

 In so far as the parasite is stable this pres- 

 sure will be constant and its effect is of 

 minor consequence ; however, whenever the 

 parasite is in a condition of active growth 

 the gradually increasing pressure becomes 

 an important element to consider, but the 

 discussion of this falls naturally under a 

 subsequent heading. 



There are certain influences which the 

 parasite exerts that are mechanical, and 

 still are not traceable to it as a mere inert 

 body; they rather are determined by its 

 activity, and yet they properly deserve con- 

 sideration under this head, for the parasite, 

 though active, is exerting a purely mechan- 

 ical influence, and one might conceive of 

 the same results following upon the move- 

 ment of any inanimate object. If the para- 

 site moves about it tends to irritate and 

 inflame or destroy the surface upon which 

 it lies, even though it remains relatively 

 fixed in location and merely twists from 

 side to side. The irritation here will be 

 evidently local and will be due to, or at 

 least emphasized by, the spines or other 

 roughness of the bodj^ It will be also in- 

 creased if the parasite possesses suckers or 

 other organs of attachment. The delicate 



