ARTERY. 



223 





maceration, the intervals between the fibres 

 become greater, and as the putrefactive pro- 

 cess sets in and advances, the whole substance 

 of the middle tunic takes on the form of a 

 spongy mass, and ultimately the fibres cease 

 to be any longer discernible, having been re- 

 duced to the state of a soft pnlp, while the 

 cellular structure is rendered more evident. 

 The following appears to us to be the rationale 

 of the phenomena above described : the in- 

 crease in thickness which the middle tunic at 

 first undergoes is owing to the cellular tissue 

 interposed between the fibres imbibing the 

 water in which it has been immersed, in virtue 

 of its hygrometric property ; and the spongy 

 appearance observable after the maceration 

 has been continued for a length of time, is the 

 result of the cellular tissue having the property 

 of resisting decomposition by putrefaction 

 much longer than the fibrous tissue. 



The internal tunic (intima of Haller) is the 

 thinnest of the three ; it is continuous with 

 the lining membrane of the heart, in extending 

 from which into the arteries it forms a dupli- 

 cature, contributing to the composition of the 

 semilunar valves: in the larger arteries, when 

 empty, it sometimes forms longitudinal folds ; 

 in some arteries, such as the popliteal, and the 

 brachial at the bend of the elbow, it presents 

 transverse folds or wrinkles; it also forms 

 transverse wrinkles in arteries which have re- 

 tracted after amputation : its internal surface, 

 which is in contact with the blood in the living 

 body, is smooth, polished, and bedewed with 

 a fine exhalation ; its external surface adheres 

 to the internal surface of the middle tunics in 

 the larger trunks of the arteries; this tunic 

 may be divided into two layers, the internal 

 of which is thin and transparent, while the 

 external is whitish and opaque, having its struc- 

 ture blended with that of the middle tunic ; 

 it is the tunica cellulosa Interior of Haller, 

 and is the seat of the calcareous, steatomatous, 

 and atheromatous deposits, which so frequently 

 occur as morbid appearances in the coats of 

 the arteries. We do not perceive fibres nor 

 any other signs of organization in the inner 

 layer of this tunic in its healthy state ; it is 

 almost completely inelastic and very brittle; 

 it tears with equal facility in every direction ; 

 compared with other structures it bears the 

 closest resemblance to the arachnoid mem- 

 brane of the brain; the smooth and highly 

 polished condition of the free surface of this 

 tunic is an admirable provision, whereby the 

 effect of friction in diminishing the velocity 

 of the passage of the blood through the arte- 

 ries is reduced to the smallest possible amount. 



The following mechanical contrivance ob- 

 servable in the interior of the arteries would 

 appear to be a provision for facilitating the 

 distribution of the blood through the divisions 

 of the arterial system. As the branches of the 

 arteries mostly arise from the trunks at acute 

 angles, the portion of the circumference of 

 their orifices on the side next the heart is 

 smooth and depressed, forming a sort of chan- 

 nel sloping gently from the trunk into the 

 branch, while the opposite side, or that more 

 remote from the heart, is bordered by a ridge 



of a semilunar valve-like form, composed of a 

 duplicature of the lining membrane in which 

 there is included a portion of the middle 

 tunic ; the more acute the angle at which the 

 branch arises, the greater is the prominence of 

 this ridge ; it is altogether absent where branches 

 arise at right angles, as in the case of the emul- 

 gent arteries, and where branches arise at ob- 

 tuse angles to the trunk, it is found at their 

 orifices on the side next the heart. 



The aorta and pulmonary artery are each 

 provided with three valves at their origins from 

 the ventricles ; these valves, called sigmoid or 

 semilunar from their semicircular form, are 

 attached by their inferior borders, which are 

 convex, to the margins of the semicircular 

 flaps or festoons, into which the edge of the 

 commencement of the middle tunic of the 

 artery is divided ; the superior edges of each 

 of these valves, which are free and floating, 

 form two concave lines, separated by a 

 projection in the centre, in which is con- 

 tained a small cartilaginous body, called 

 tubercle, globulus Arantii or corpus sesa- 

 moideum. The portions of the walls of the 

 artery corresponding to the valves are dilated 

 in the form of pouches, more marked in the 

 aorta than in the pulmonary artery ; these are 

 the sinuses of Valsalva. The semilunar valves 

 are composed of a duplicature of the lining 

 membrane of the artery, including within it a 

 thin but strong fibrous expansion, continuous 

 with the fibrous structure, which connects the 

 middle tunic of the artery with the tendinous 

 ring encircling the arterial opening of the ventri- 

 cle; the free border of each valve contains a small 

 fibrous cord, as described by Beclard, having 

 the globulus Arantii attached to it in its centre. 

 An increase or diminution in the number of 

 the sigmoid valves is of rare occurrence, more 

 frequently presented in the pulmonary artery 

 than in the aorta, and oftener consists in the 

 number of valves being increased to four than 

 diminished to two.* 



The mechanism of these valves is such as to 

 prevent the blood flowing in a direction con- 

 trary to its regular course ; for when that fluid 

 is propelled towards the ventricle, they are 

 separated from the parietes of the artery, and 

 being distended by the column of blood pres- 

 sing against their superior surfaces, they are 

 laid across the area of the vessel, which they 

 completely fill up by their edges being thus 

 brought into perfect contact and the globuli 

 Arantii meeting in the centre. There are no 

 valves in the arteries in any other situation. 



The arteries, like other organized struc- 

 tures, are furnished with proper nutritious 

 arteries and veins called vasa vasorum. The 

 aorta and pulmonary artery at their commence- 

 ment receive some branches from the coronary 

 vessels of the heart ; in all other situations the 

 vasa vasorum are supplied by the neighbouring 

 bloodvessels ; the vasa vasorum are very evi- 

 dent in the external tunic of the arteries, they 

 can be traced until they penetrate the sub- 

 stance of the middle tunic, but not farther ; 



* Meckel, Handbuch der menschlichen Anato- 

 mic. Band. i. 



