ANATOMY. 



The pulmonary artery, which arises 

 from the upper and anterior part of this 

 ventricle, conveys the blood into the 

 lungs. The opening of this artery, which 

 is called the arterial orifice of the ventri- 

 cle, is furnished with three valves, called 

 sigmoid or semilunar, which prevent any 

 retrograde motion of the blood from the 

 artery towards the heart. 



The venous blood, by being exposed to 

 the atmospheric air in the lungs, is alter- 

 ed in its properties, and becomes arterial 

 blood, in which state it is returned to the 

 left auricle of the heart by four pulmo- 

 nary veins, two of which belong to each 

 lung. This left or posterior auricle con- 

 sists of a large cavity, called the sinus of 

 the pulmonary veins ; and of a smaller 

 process or auricula. It is situated quite 

 at the upper and back part of the heart, 

 and ^ransmits the blood through the au- 

 orifice of the left ventricle into 

 th;V, cavity. This opening is perfectly si- 

 milar in all essential circumstances to the 

 corresponding part on the right side of 

 the heart But its valve, being divided 

 into t-vo portions only, is called mitralis, 

 from a comparison with a bishop's mitre. 



The left ventricle is much thicker and 

 stronger than the right. It feels exter- 

 nally almost like a solid mass of flesh ; 

 while the right is comparatively thin and 

 flabby. The reason of this difference is 

 obvious. The left ventricle has to drive 

 the blood to the most remote parts of the 

 body, whereas the right only sends it 

 through the lungs. The aorta arises from 

 the left ventricle, and its mouth is guard- 

 ed by three semilunar valves. This is the 

 trunk from which the arteries of the 

 whole body arise. 



STRUCTURE OF THE ARTEIUES. 



Those vessels, through which the blood 

 flows from the heart into every part of 

 the body, are called arteries. The term, 

 which is derived from cttjp, air, and rtjpe a, 

 I hold, was first adopted by the anato- 

 mists of the Alexandrian school, in con- 

 sequence of the erroneous opinion which 

 they entertained, that these vessels were 

 designed for the distribution of air 

 throughout the body. 



The larger arteries have thick and 

 clastic sides, so that they remain open 

 when divided, and present a regularly 

 circular aperture. The sides may be se- 

 parated into three strata of dissimilar 

 substances, which are technically called 

 coats. The innermost, which is generally 

 termed the cuticular coat, is thin, strong, 



and highly elastic. The internal surface 

 of this coat is perfectly smooth, so that 

 the blood glides along it without impedi- 

 ment ; the external surface is connected 

 to that coat which surrounds it. The 

 middle, or, as it is called, the muscular 

 coat, is composed of a congeries of cir- 

 cular fibres, separable into numerous 

 strata, but not much resembling muscu- 

 lar fibres as found in other situations. 

 The external coat of the artery is made 

 of condensed cellular substance, which 

 unites these vessels to the neighbouring 

 parts. 



It appears that the larger vessels have 

 the greatest elastic power, with the small- 

 est muscular force ; while these proper- 

 ties exist in reversed proportions in the 

 smaller vessels. In the large arteries 

 muscular power is unnecessary, for the 

 force of the heart is fully adequate to the 

 propulsion of the blood; but in the small- 

 er arteries, where the effect of the heart's 

 action declines, a pi-oportionate muscular 

 power is allotted to the vessels, to urge 

 on the circulating fluids. 



The arteries have their nutrient arte- 

 ries and veins, their absorbents, and their 

 nerves. 



All the arteries proceed from one great 

 vessel, as the branches, spring from the 

 trunk of a tree ; and we proceed to no- 

 tice certain circumstances observable in 

 their ramifications. 



1. When an artery gives off a branch, 

 the conjoined areas of the two vessels 

 make a greater space for the blood to 

 move in, than the area of the original ves- 

 sel. The increase of dimensions in the 

 branches of a large artery is slight; but in 

 those of a small one it is so considerable, 

 that Haller has estimated it as surpassing 

 by one third that of the trunk from which 

 they sprung. The conjoined areas of all 

 the small arteries so greatly exceed that 

 of the aorta, that the same anatomist, in 

 opposition to former opinions, affirms that 

 these vessels are conical, the basis of the 

 cone being in the extreme arteries, and 

 the apex in the heart. 



2. When a large artery sends off a 

 branch, its course does not in general de- 

 viate further from that of the trunk than 

 an angle of 45 degrees. Sometimes a 

 branch, which has gone off at an acute 

 angle, returns, and proceeds in a contrary 

 direction to that of the trunk. Sometimes 

 indeed a large artery does proceed from 

 the trunk at nearly a right angle, as the 

 renal arteries. Though the large arteries 

 generally ramify at acute angles, there is 

 great diversity in the branching of thr 

 smaller ones. 



