149 



elasticity, it might likewise be occasioned by the contraction of fibres 

 not absolutely circular nor longitudinal, but spiral and imperfectly sur- 

 rounding the vessel, and crossing each other in various directions. This 

 yellow coat, thicker in proportion in the smaller arterial twigs, than in 

 the larger branches, and thicker in these than in the trunks, is dry, hard, 

 not capable of much extension, and is ruptured by an effort to which the 

 external coat yields by stretching. Lastly, a third, thin, and epidermoid 

 coat lines the inside of these vessels, and seems less adapted to give 

 strength to the parietes of the arteries, than to facilitate the flow of the 

 blood, by presenting to it a smooth, even, and slippery surface, continu- 

 ally moistened by a serous exudation, from the minute arteries, or vasa 

 vasorum, which are distributed between these coats*. 



Besides these three coats, the great arteries receive a fourth from the 

 membranes lining the great cavities; thus, the pericardium and the 

 pleura in the chest, the peritoneum in the abdomen, furnish to the dif- 

 ferent parts of the aorta, an adventitious coat which does not completely 

 surround the vessel. 



Of the three coats which form the parietes of the arteries, the fibrous, 

 though thicker than the other two, offers, however, the least resistance. 

 If you take the carotid artery, which for a considerable space, does not 

 send off any branches, and forcibly inject into it a fluid, the internal and 

 middle coat will be torn, before dilatation has increased, by one-half, the 

 calibre of the vessel. The external coat resists the cause of rupture, 

 by dilating, and forms a tumour, and it is only by applying a conside- 

 rable force, that it can be ruptured. The experiment is attended with 

 the same success, if performed with air or any other gas. In aneu- 

 rism, the internal and fibrous coats of the arteries, but more particularly 

 the fibrous, are ruptured at an early stage of the disease, which at that 

 period increases suddenly, in a very rapid manner, and on opening the 

 tumour, it is observed, that the sac is entirely formed by the dilated cel- 

 lular coat. Take an artery of a certain calibre, for example the carotid 

 or humeral, apply a ligature around it, and tighten it with some degree of 

 force. Dissect and take out the vessel, then cut the thread, and examine 

 the place to which it was applied, you will observe, that the parietes of 

 the artery are in that part thinner, and formed merely by the cellular 

 coat which alone has withstood the constriction. Take hold of the two 

 ends of an insulated arterial tubt, and stretch it, then examine its inner 

 coat, and you will find it torn and cracked in several places, and the pa- 

 rietes of the artery evidently weakened!- 



LVI. This want of extensibility in the coats of arteries, is the princi- 

 pal cause of aneurism ; hence the popliteal artery is so liable to that af- 

 fection, from its situation behind the keee, whose extension is limited, 

 merely by the resistance of the posterior tendons and ligaments: this ar- 

 tery is affected by the jar which takes place through all the soft parts, 

 when the leg is violently extended; and being less extensible than the 

 other parts, its inner coat is ruptured, or at least weakened, so as to oc- 

 casion an aneurism, always rapid in its progress. Of ten popliteal aneu- 

 risms which I have seen in different hospitals, eight were ascribed to a 



* See APPENDIX, Note R, for remarks on the structure and action of the arteries. 

 7 For a full relation of the effects of ligatures on arteries, see Jones on Hemorr- 

 hage." Godman, 



