258 ClllCULATORY APPAIIATLS. 



1st, hypertrophy of the left ventricle; 2iid, dilatation of the 

 affected ^'esseIs. Each of these changes tends to compensate for 

 the primary lesion ; hence it is that the hy})ertrophy is inversely 

 pro})orlionate to the dilatation. 



b. Dilatation of the Vessels. 

 a. Dilatation of Aj-teries. — Aneurisrii. 



§ 22o. Attempts to introduce order among the infinite variety 

 of forms presented by true aneurisms, i.e. aneurisms involving 

 all the three arterial coats, used at one time to be based on the 

 shape of the dilated vessels as a fundamentuni divisionis ; this 

 led to the distinction of cylindrical, saccular, fusiform and vari- 

 cose aneurisms. These attempts all failed. We should be more 

 successful could we make use of the etiology, the mode of origin 

 of the various aneurisms, as a clue to the diversity of the indi- 

 vidual forms. But the etiology of aneurism is unfortunately 

 ■N'ery obscure ; and it may be long before it is finally cleared up. 

 Hypotheses indeed abound. E.g. nothing seems more tempting 

 than to seek for the origin of the mischief in a lack of elasticity 

 or of contractile power in the muscular coat. Of all the com- 

 ponent elements of the arterial wall the muscular fibres are 

 those in which nutritive change is most active, and which, we 

 may fairly assume, are most susceptible of nutritive disturbance. 

 Hence I willingly take refnge in the hypothesis of atony of the 

 middle coat, to explain the uniform cylindrical or varicose dilata- 

 tion of all the larger arteries which is frequently met with in 

 old people, without the presence of any visible alteration in their 

 walls. I likewise admit that it may have some, though indeed 

 a less important share in the production of other forms of dilata- 

 tion. The hypothesis of a simple disturbance of innervation, a 

 paralysis of the muscular coat (Eokitanski) is less widely appli- 

 cable. Here too however there is at least one case in which this 

 explanation commends itself to us. I refer to the anastomotic 

 variety of aneurism. The term denotes a varicose dilatation, 

 elongation and twisting of the trunk and all the branches of an 

 artery — the ectasy of a limited section of the arterial system* 

 It occurs chiefly in the arteries of the scalp, e.g. the occipital 

 and temporal arteries, and reminds us too forcibly of that dilata- 



