256 CIRCULATORY APPARATUS. 



rism of that vessel. This accident is usually regarded as simply 

 a result of the concomitant hypertrophy of the left ventricle. 

 The rent is usually situated about a finger's-breadth above the 

 semilunar valves ; it extends at first only through the internal 

 and middle coats ; the adventitia is peeled from the media ; the 

 blood tunnels its way between them, upwards as far as the heart, 

 downwards as far as the descending aorta ; it never gets as far as 

 the abdominal aorta. When the sac thus formed is stretched to the 

 utmost, a second rupture occurs, either outwards into the medias- 

 tinum, or (a rare event) inwards into the aorta at another point, 

 so that for a short distance, the blood travels along two parallel 

 canals. On examining the edges of the ruptured intima we occa- 

 sionally find them in a perfect state of fatty degeneration ; further 

 investigations however are required, before we can decide upon 

 the frequency and causal moment of this complication. 



§ 221. Another, and a tolerably constant complication of 

 chronic endoarteritis is calcification of the middle coat. This is 

 not so prevalent in those parts of the arterial system which are 

 chiefly predisposed to endoarteritis (such as the aorta and its 

 main divisions), as it is in the relatively thick muscular coat of 

 the smaller and smallest arteries, in those of the arm and leg, in 

 the temporal arteries with their branches. Here too the process 

 is one of simple petrifaction ; calcareous salts are deposited in 

 the muscular fibre-cells, which reappear unaltered when the 

 salts are removed by acid. Even without the aid of the micro- 

 scope the probable seat of the calcification may be determined ; 

 since the distinctly annulose aspect of the infiltrated parts can 

 only be attributed to the transverse arrangement of the muscular 

 fibres (fig. 82). 



Calcification of the middle coat, as an independent affection, 

 apart from atheromatous change, is more rare. In cases of 

 calcareous metastasis (cf. § 51) all three coats of the vessel are 

 uniformly infiltrated as a rule. 



§ 222. Let us now inquire what influence the above-described 

 alterations in the walls of the vessels may exert upon the move- 

 ment of the blood through them. These alterations may be sum- 

 marily classed under two heads : 1st, narrowing of the calibre 

 of the vessel ; 2nd, rigidity of its tube. This rigidity, mainly 

 due to calcification of the intima as well as of the media, causes 

 a certain proportion of the vis a tergo of the heart to be in some 



