VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS. 233 



artery, or a distension of it without a rupture, or a rupture of the internal coats 

 of the artery, and a distension of its external coat. 



A rupture of the large branches of the aorta necessarily allows so plentiful 

 effusions of the blood, as to occasion immediate death ; while the capillaries 

 may be burst without any other injury, but a slight ecchymosis, and the tumour 

 formed by the effusion from them will be diffused and superficial. 



A rupture of the mean branches, or such as descend between the tibia and 

 fibula, the radius and ulna, &c. will be attended with a considerable effusion of 

 blood ; but as the blood will find a passage between the interstices of the 

 muscles, it will never form a circumscribed tumour. However, the effusion 

 being continued per saltum, through the ruptured artery, will give a faint 

 pulsation, and consequently some resemblance of the aneurism ; for which 

 reason it is by some surgeons termed a bastard aneurism. 



Whether an aneurism be a tumour formed by the dilatation of the artery, or 

 by a rupture of the internal coats of the artery, and a distension of the external, 

 has for some time been a matter of great dispute ; each party protesting, per- 

 haps too unjustly, against the possibility of the other's opinion. As to the 

 possibility of an artery's being dilated, it stands supported by reason and autopsy. 

 We find the uterine arteries constantly increased in thickness and diameter, in 

 proportion as the uterus is distended ; and many cases of palpitations of the 

 heart have been attended with great dilatations of the aorta; instances of which 

 have occurred both in human and brute subjects. Such a dilatation will neces- 

 sarily follow a constant, or frequent pressure on any part of the aorta, provided 

 such pressure does not entirely stop the progressive motion of the blood through 

 the aorta. 



But on the other hand, such a dilatation will always retain somewhat of the 

 form of the artery. The resistance will not be every way equal, as in the 

 extravasate tumours ; because the quaquaversal pressure of the blood will be 

 controlled by the pressure on the artery, and the resistance from the coats of 

 the arteries, so as necessarily to form a cylindroid. And the consequence of 

 such a dilatation cannot, if considered abstractedly from its pressures, be worse 

 than from a varicose vein, if so bad. 



Again, those who conceive an aneurism to be a rupture of both coats of the 

 artery, oppose their opinion, who imagine the internal coat to be ruptured, and 

 the external to be distended, by comparing the two coats in question, and 

 urging, that as the internal coat is so much thicker than the external, it seems 

 impossible the last should be sufficient to resist a force capable of destroying the 

 first. Were these two coats similar as to their structure, we might then com- 

 pute their strength by their thickness, and this argument would be of much 



VOL. VII. H H 



