Arteries and Their Transformations
In arteries, as already indicated, the blood flow is away from the heart, making the outward delivery trip of the things needful from the source of supplies to the capillaries. This object is attained in all vertebrates by means of one fundamental scheme of pathways, although the general plan is modified to meet the demands of different types of vertebrates. Once the main trunk line of the dorsal aorta emerges from the complexities of the arterial arrangement in the head and gill region, further distribution to the various organs of the body of arteries branching off from the aorta is fairly uncomplicated and uniform throughout the vertebrate series. The anterior arteries of the gill region that bear the brunt of the transformation from water to land life, however, show greater diversity.
In amphioxus the ventral aorta, or the main blood vessel anterior to the liver diverticulum, is connected with the dorsal aorta by a number of pairs of lateral loops that encircle the anterior part of the digestive tube. There may be as many as sixty pairs of these loops in the adult animal, occupying a large portion of the anterior part of the body. Each pair is interrupted by the insertion of gill capillaries in which the blood takes on oxygen from the surrounding water.
In true vertebrates the number of pairs of branchial loops, although less in the adult, is typically six during embryonic development, with the exception of certain primitive sharks, for example Hexanchus which has seven, and some cyclostomes which exceed the typical number. The first pair is laid down as part of the original vitelline circulation. Subsequently five additional pairs are laid down one after the other, beginning with the most anterior one (Fig. 300).
The usual embryonic arrangement of these branchial arterial vessels, as indicated in Figure 301, may be taken as a point of departure for the adaptations to follow in the different vertebrate classes.
It will be seen that the branchial loops do not connect directly with the main dorsal aorta but first join with two smaller blood vessels, the radices aortae, which secondarily join, like the converging arms of the letter Y, to make the single dorsal aorta.
In fishes (Fig. 302), the two most anterior pairs of loops, that are supported by the mandibular and hyoid arches of the splanchnocranium, are reduced to branches of the third in adult life, leaving the remaining four to become the branchial arches, interrupted by the capillaries of the internal gills as they pass from the ventral side to the radices aortae.
In urodele amphibians (Fig. 303) external gills are introduced which, unlike the internal gills of fishes, do not directly interrupt the branchial loops but are established on a detour from the loops, so that it is possible for the blood to pass from the ventral to the dorsal aorta by either of two routes, one through the uninterrupted branchial loop in which no capillaries are present, and the other by way of a side line through the capillaries of the external gills.
Three pairs of such external gills, situated on loops IV, V, and VI, may be present in urodeles. Thus in those salamanders that discard their gills during metamorphosis, it is possible for the blood to progress without interruption by way of the branchial loop direct, avoiding the disastrous consequences which would inevitably result if a single unavoidable route through the internal gills, as in fishes, were put out of commission.
In urodeles, it should be noticed that the last and most posterior loop (VI) has its ventral portion pressed into the service of the pulmonary artery which goes to the newly-established lungs, for the oxygen-supply, instead of directly to the dorsal aorta. The dorsal part of this loop is reduced to a ductus arteriosus, or duct of Botallus. In all amniotes this duct functions until hatching or birth and its ghostly remains still haunt the arterial complexes of higher vertebrates, even of man, serving as a reminder of emergence from water to land life. Furthermore, in urodeles loop V and that part of each radix aortae between loops III and IV, in anticipation of their later obliteration in anurans, become much attenuated.
The anurans, represented by the frog, pass through a youthful tadpole stage in which their branchial arteries resemble those of urodeles, but they go a step further in burning their ancestral arterial bridges behind them, since only three pairs (III, IV, and VI) of the six original embryonic loops survive in the adult. Loop IV becomes the large graceful paired systemic arch, while loop III is entirely devoted to supplying the head region. Since the connectives of the radices aortae that run between loops III and IV are suppressed, the blood in the third loop can no longer pass backward directly into the dorsal aorta. Loop V, already showing signs of degeneration in the perennibranchiate urodeles, disappears entirely in adult frogs, as does the dorsal part of loop VI, the ventral part of which persists as the pulmocutaneous artery.
In reptiles three pairs of loops, namely, III, IV, and VI, survive (Fig. 304). With the beginning of a separation of the ventricle into two parts, the ventral aorta near the heart splits, not into two but into three parts, two of which go with the right ventricle and the third with the left one. Of the portions draining the right ventricle, one connects with only loops VI to become the pulmonary aorta while the other leads into the left aortic arch IV as the left systemic aorta. The division of the ventral aorta draining the left ventricle sends blood forward into the carotids and also through the right fourth loop, right systemic aorta.
In amphibians the mixture of aerated and non-aerated blood occurs in the single ventricle of the heart before it is sent out over the IVth arterial loop, but in reptiles, which have at least a partial partition established between the ventricular chambers of the heart, the mixing of “pure” and “impure” blood may lie postponed until the right and left branches of loop IV pour their diverse contributions into the common dorsal aorta. Reptiles as well as amphibians are “cold-blooded,” one contributing reason being that in both cases some of the blood that has not been oxygenated is poured back into the dorsal aorta and sent again “unpurified” over the body. The result, like mixing clinkers with coal, is that the fires do not bum any too brightly and cold-bloodedness follows.
Birds, which arose from an advanced reptilian stock, follow the general reptilian pattern but they discard the left systemic aorta (Fig. 305). Thus the right fourth loop and accompanying radix aortae form the single systemic arch. This important change results in all of the blood pumped by the right ventricle going to the lungs, none of it becoming mixed with the blood which is sent out from the left ventricle to all of the other organs of the body.
Mammals, derived from a primitive reptilian stock before major modifications in the direction of the modern reptilian plan, have both right and left fourth loops connected with the portion of the ventral aorta leading from the left ventricle (Fig. 306). In mammals it is the left fourth loop and accompanying radix aortae which persist as the systemic arch while the right fourth loop becomes the beginning of the right subclavian artery.
Thus in man a single large aortic loop, like a shepherd’s crook, arises from the heart, arches over to the left, and passes backward to supply the body and its various organs. This loop is the combined product of (1) the embryonic ventral aorta; (2) the left side of the IVth branchial loop; and (3) the left arm of the radices aortae.
Occasional rare cases, reported in medical literature, of double aortic arches in man, or of the aortic arch on the right side as in birds, find a ready interpretation in the light of comparative anatomy.