2bO ClilCULATOllY ArrARATtJS. 



tliiiL thruuglioufc the whole arterial .sybtem, a jerky propulsion of 

 the mass of blood has taken the place of a rhythmic acceleration 

 of its current. The brief though forcible shock, which the 

 radial artery communicates to our finger, will be most intense in 

 the aorta itself, since there the loss of energy by way of impact, 

 which necessarily attends this mode of propulsion, does not yet 

 besin to tell. Therefore it is that aueurismal dilatation is most 

 common, first in the ascending aorta ; and secondly, wherever 

 the local narrowing of an artery causes a special rise of pressure 

 just behind the obstruction. With special reference to this point, 

 1 have examined all the peripheric aneurisms to which I could 

 get access, and I have rarely failed to discover a more or less 

 marked thickening of the intima below the dilated part, as the 

 exciting cause of the lesion. Persistent contraction of those 

 muscles among which the main branches of an artery are dis- 

 tributed, may also cause aneurismal dilatation of the parent- 

 trunk. Thus for example aneurism of the popliteal artery not 

 unfrequently rewards the exertions of those lackeys who stand 

 for hours together behind their masters' carriages (contraction of 

 gastrocnemii). 



§ 225. As regards the outward form of aneurisms, the uni- 

 formly cylindrical or fusiform dilatation of the arterial tube is 

 usually distinguished from the one-sided, saccular protrusion of 

 its coats. The cylindrical aneurism becomes an invaghiating 

 one when the aneurismal sac, at one or both of its extremities, 

 overlaps the corresponding ends of the undilated part of the 

 vessel. The saccular variety presents similar modifications. 

 Should the sac overlap the parent-vessel on one side only, its 

 ca^ ity is marked oft' from the interior of the vessel by a pro- 

 jecting semilunar fold; should it overlap its parent-artery both 

 i'lbove and below, or on all sides at once, it is said to have ^^a 

 neck." 



Saccular aneurisms, those which project more from the convex 

 than from the concave side of the ascending aorta — are by far 

 the most common. They press from within against the sternum 

 or the sterno-clavicular joint. Aneurisms of the transverse arch 

 also spring more often from the convexity than from the con- 

 cavity of the vessel. Here too we not unfrequently find aneu- 

 risms with narrow necks which ultimately force their way out 

 through the upper aperture of the thorax and invade the oeso- 



