Collection of facts illustrative of the morbid conditions of the pulmonary artery / by Norman Chevers.

  • Norman Chevers
Date:
1851
Catalogue details

Licence: Public Domain Mark

Credit: Collection of facts illustrative of the morbid conditions of the pulmonary artery / by Norman Chevers. Source: Wellcome Collection.

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    Normal Anatomy of the Pulmonary Artery. This is a vessel sui generis, partaking al¬ most equally of the structural characteristics of the arteries and the veins ; resembling the former in the character and arrangement of its principal tissues, and the latter in its pliability, thinness, and dilatability. As the distance which the blood has to be propelled through the pulmonary vessels is short compared to that which it traverses in permeating the extended tract of the syste¬ mic arteries, and as the entrance of the ve¬ nous blood to the lungs is greatly aided by the respiratory movements, the distending power which the right ventricle ordinarily exercises upon the pulmonary artery is con¬ siderably less than that of the left ventricle upon the aorta. Still, the muscular force of the right ventricle, in a state of health, is by no means inconsiderable, and the shortness of the tract of the pulmonary artery, toge¬ ther with the frequent causes of transient obstruction to which it is almost necessarily liable, render it requisite that it should be constructed in a manner which will at once enable it to escape rupture, to undergo con¬ siderable distension, and to return to its natural condition immediately after the cause of over distension shall have ceased to ope¬ rate. It is evident that a vessel possessing the comparatively rigid and inelastic structure of the aorta would, if occupying the position of the pulmonary artery, very shortly become inefficient under the frequent causes of tem¬ porary and permanent obstruction to which that vessel is exposed—its strong and closely knit fibres would soon become stretched and inelastic, its inner tissues would become thickened and loaded with fibrinous and bony deposits (the necessary concomitants of great dilatation in the systemic arteries), and, in a few years, aneurism, or laceration of its coats, would almost certainly result. Hence we find that the pulmonary artery is constructed in a, manner which renders it capable of long resisting the injurious ef¬ fects of over-excited cardiac action and pulmonary obstruction. The recurved form of its branches in great measure prevents the impulse of the ventricle from being too powerfully exerted upon the minute vessels of its subdivisions; and the thinness, plia¬ bility, and tenacity of its structures, fit it to receive powerful shocks with impunity, and to suffer extraordinary distension without at once becoming permanently dilated. I have been at some pains to determine the relative dilatability of the aorta and the pulmonary artery, and have found, by ac¬ curate measurements, that the ascending portion of the latter is very far more dila¬ table laterally (or when its tissues are stretched in the direction of the fibres of its middle coat), than is the latter at the commencement of its arch ; while, although the structures of the pulmonary artery are somewhat more extensible in a vertical di¬ rection than are those of the main sys¬ temic trunk, the difference in this respect between the two vessels is far less consider¬ able. The aorta, on the contrary, although less yielding in every way than the pulmonary artery, is more extensible in the vertical than in the horizontal direction.* This might be anticipated from our know¬ ledge of the anatomical relations and the functions of the parts. The entrance of the blood to the pulmonary artery being, iu great measure, due to the action of the lungs, and the right ventricle possessing, under ordinary circumstances, a very mode¬ rate degree of muscular power, the vessel does not require to be possessed of remark¬ able extensibility in the line of its axis, but the frequent delays to which the circulation through the lungs is liable render it abso¬ lutely necessary that it should be capable of great lateral distensibility, so as to perform, during brief periods, the office of a tempo¬ rary reservoir for the blood, which can neither return to the ventricle, nor gain a ready transit through the lungs. The aorta, on the contrary, is, under ordinary circum¬ stances, far less liable to causes of tempo¬ rary obstruction. During periods of great muscular effort, &c. the passage of blood through its branches is somewhat delayed, but this retardation occurs to a less degree, and operates within a more extended system of vessels, than is the case in the pulmonary artery ; hence remarkable dilatability is not requisite in the aorta ;—on the other hand, however, the impulse of the powerful left ventricle acts with great extending force upon the whole aortic tract, and therefore it becomes necessary that the tissues of this vessel should possess great elastic extensi¬ bility in the vertical direction. In a previous essayf I have described the beautiful arrangement of a structure situated between the epithelial lining and the elastic coat of the arteries, and which appears to be in great measure designed to assist them in resisting undue vertical extension. This * In one of my experiments, I took a square portion of the aorta of a young adult from the sinus of Valsalva,and another portion, of precisely equal dimensions, from the ascending pulmonary artery (their structures were perfectly free from any morbid change), and I found their relative dilatability and extensibility to be as follows Taking the transverse and longitudinal measure¬ ments of both portions as 27, the lateral dilata¬ bility of the aortic portionvf&s represented by 414 > that of the pulmonary by 47. The longitudinal extensibility of the aortic fragment was equal to 43 ; that of the pulmonary, to 45. t Account of the Sub-serous Membrane of the Aorta, Guy’s Hospital Reports, No. 10.
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