The channels by means of which the blood is collected and returned to the heart have undergone a greater degree of evolutionary adaptation in the vertebrate series than the corresponding arteries that distribute the blood over the body from the heart. This is due in part to the elaborate complexes of capillaries inserted in the course of the veins which form the “portal systems,” and in part to the accessory services of the lymphatic system of channels.
As would be expected the least complicated arrangement of veins is found in amphioxus (Fig. 307). Blood from the body wall is picked up by paired anterior and posterior cardinal veins which communicate with the ventral aorta by means of the common cardinal veins, or ducts of Cuvier. An unpaired caudal vein picks up blood from the postanal region of the body and, near the anus, joins both the right postcardinal and the subintestinal vein, which continues forward beneath the intestine, not only receiving contributions from the capillaries that encircle the alimentary canal, but also deriving food from the canal itself.
Upon reaching the liver diverticulum the subintestinal vein breaks up into capillaries, thus establishing a primitive hepatic portal system. The hepatic vein, from the liver, is soon joined by the common cardinals to form the ventral aorta, which carries blood to the gills and is destined to evolve into the heart.
Development of Veins in Elasmobranchs
A brief survey of the main steps in the development of the veins in elasmobranchs will aid us in a better understanding of the adult plans of not only the lower fishes but vertebrates in general. At least seven of these major steps may be recognized, namely, (1) vitelline stage, (2) subintestinal stage, (3) common cardinal stage, (4) hepatic portal stage, (5) subclavian stage, (6) subcardinal stage, and (7) renal portal stage (Fig. 308).
A pair of vitelline veins entering the body from the yolk sac, as described in section IV, is the first evidence of the venous system. They run forward ventral to the digestive tract, passing first along the duodenal region where the liver is soon to grow out ventrally.
Soon a subintestinal vein, usually connecting with the left vitelline vein, develops along the ventral side of the posterior part of the intestine and into the tail, after looping around the cloacal region.
Paired common cardinal veins, or ducts of Cuvier, each formed by the union of an anterior cardinal from the head region and a posterior cardinal from the body wall posterior to the heart, next appear. At this time anastomoses between the vitelline veins form two venous rings, the anterior one looping around the small liver diverticulum.
As the liver outgrowth increases in size it pushes into the anterior vitelline loop, gradually breaking up the vessels here until they are reduced to capillary size. In this manner there is established an hepatic portal system which in this stage begins in capillaries in the tail region as well as in the wall of the digestive system. The anterior parts of the vitellines, which carry blood from the liver to the sinus venosus, now become known as the hepatics.
The next new vessels to develop are the subclavian veins, leading from the anterior fin buds to the common cardinals, and the ventral abdominal veins running along the ventral abdominal wall to empty into the subclavians. Meanwhile the posterior cardinals have grown back until they have joined with the cloacal loop and the subintestinal vein has subsequently broken away from this loop. The hepatic portal system is now limited to the draining of the digestive system, as in adult animals.
Between the developing mesonephroi there next appears a pair of subcardinal veins, each with numerous lateral connections between it and the corresponding posterior cardinal. Gradually portions of all except the most anterior pair of these connections are broken up into capillaries in the mesonephroi.
Finally, the posterior parts of the posterior cardinals become the renal portal veins as they break away from their anterior parts. As a result of this separation the adult plan of veins associated with the mesonephroi is established. The renal portal system, composed of the caudal vein and two renal portal veins, takes blood from the tail to the mesonephroi. Blood from the mesonephroi is carried to the heart by the veins known in adult animals as the posterior cardinals, despite their embryonic origin from several sources, including a pair of subcardinals and a pair of lateral connections as well as the anterior parts only of the embryonic posterior cardinals. Meanwhile the ventral abdominal veins, growing posteriorly, have extended into the posterior fin buds as the iliac veins.
By these various steps the adult plan of elasmobranchs has been reached (Fig. 308g). Two pairs of large veins enter the heart, namely the hepatic veins from the liver and the common cardinal veins, into which empty the posterior cardinals, anterior cardinals and subclavians. From the tail region the renal portal system carries blood to the mesonephroi where it passes through capillaries and is picked up by the posterior cardinals which also drain the dorsal part of the trunk region. The anterior cardinals, aided by much smaller jugulars, return blood to the heart from the head. The subclavians drain not only the pectoral appendages and neighboring body wall, but also, through the ventral abdominals, the ventral body wall and, through the iliacs, the pelvic appendages. Blood from the digestive system is picked up by the hepatic portal system, carried to the liver where it is strained through capillaries, and picked up by the hepatic veins.
In the larger group of bony fishes the venous arrangement is much like that of the cartilaginous elasmobranchs, except that the ventral abdominal veins disappear, and the traffic from the body wall and pelvic fins is shunted over to the posterior cardinals.
Salamanders and frogs, as representative amphibians, present further stages in the evolution of vertebrate veins, which can be interpreted by comparison with the more generalized arrangement already described for elasmobranch fishes.
The plan of the principal veins in salamanders, as shown diagrammatically in figure 309A, presents three striking innovations. First, there appears a new vein, the postcava or vena cava posterior, that rivals the anterior portions of the posterior cardinals, collecting their blood from the mesonephroi. Arising embryonically from the hepatic veins, this important blood channel grows back through the liver and then along the dorsal body wall near the dorsal aorta to join the postcardinals where they fuse near their junction with the subcardinals, at the anterior ends of the mesonephroi. It increases in size until it takes over most of the transportation from the abdominal cavity to the heart.
Secondly, the iliac veins fork, each sending one branch to its renal portal, while the other branch, representing the ventral abdominal vein of the elasmobranch, fuses with its fellow to form a median ventral abdominal vein that empties anteriorly into the hepatic portal vein. Blood returning from the hind legs of an amphibian, therefore, may pass through either the renal portal or the hepatic portal capillary strainer before reaching the heart, whereas in fishes the iliac blood goes directly to the heart by way of the ventral abdominal veins without portal interference of any kind.
Thirdly, amphibians, as lung breathers, develop a pair of pulmonary veins. The pulmonary veins are not represented in these figures which show only the veins emptying into the sinus venosus or right auricle. Because amphibians use the skin to a considerable extent as a supplementary breathing organ, they have in addition a pair of well-developed cutaneous veins, from the skin, which join the subclavians. Diagrammatically these bear a superficial resemblance to the lateral veins of elasmobranchs but should not be confused with them.
The arrangement of the veins in a frog embryo, or tadpole, is like that of a salamander, except that the rivalry between the newly established postcava and the diminishing posterior cardinals culminates in the case of the frog tadpole in the successful monopoly of the circulatory blood traffic by the former and the disappearance of the latter (Fig. 309B). With the elimination of the posterior cardinals, the anterior cardinals become single, continuous channels with the ducts of Cuvier, forming veins now called precavas (venae cavae anteriores) into which the jugular veins empty to return blood from the head directly into the right auricle of the heart. In the frog the iliacs are represented by femoral and sciatic veins, the femoral splitting to enter the renal portal (postcardinal) and abdominal, and the sciatic entering the renal portal. Though the tail is lost, there is no consequent loss of the renal portal. Such drastic, changes as these, designed to meet the difficult conditions accompanying the precarious transitional method of their life, are typical of the many bodily makeshifts which this small struggling group of vertebrates has had to resort to in order to accomplish the great evolutionary feat of emerging from water to land.
The degenerating posterior cardinals become replaced in reptiles by a pair of longitudinal vertebral veins (Fig. 310A), that involve anastomoses of inter segmental and intercostal veins.
In the head region of lizards, snakes, and turtles, as pointed out by Bruner, the venous system is characterized by an abundance of sinuses or blood-filled enlargements of the veins, both inside and outside of the cranium. Through a modification of the blood pressure in the superficial sinuses that extend over the skull beneath the skin, the molting (ecdysis) of the corneal layer over the head is facilitated. Recourse to such a loosening device as these sinuses is advantageous in the case of these reptiles because their thick dry integument, which is particularly tight over the head, does not easily allow for ecdysis. In Phrynosoma, the “horned toad” of the cactus regions of southwestern United States, the venous sinuses, together with associated muscles, form a curious protective mechanism whereby these grotesque animals under excitement are able to squirt blood from their eyes by way of ruptured sinuses of the orbital veins. Modifications of the head veins are much less in evidence in the Crocodilia than in the three lower groups of reptiles just mentioned. In fact the Crocodilia deviate markedly in many particulars from all other living reptiles and may possibly be regarded as anatomically the evolutionary advance-guard of the reptilian army.
The renal portal system, although possibly persisting in modified form in reptiles, disappears in birds, with the loss of a muscular tail, while the abdominal vein of amphibians and reptiles merges in birds into an epigastric vein (Fig. 310b), which is possibly homologous with the embryonic umbilical vein of mammals.
The lost renal portal system is not recovered in the higher vertebrates, even in those species that possess well-developed tails (Fig. 311).
The venous system of mammals is further characterized by the introduction of certain novelties. The persisting anterior end of the right posterior cardinal vein, together with remnants of the transient supracardinal and subcardinal parallel to it, becomes the azygos vein, while a fragment of this complex on the left may join forces as the hemiazygos vein in the more posterior part of the body. The azygos and the hemiazygos veins are connected by transverse anastomosing bridges into the azygos system.
In marsupials, rodents, insectivores, and many artiodactyls, the azygos system is about equally developed on the two sides, while in the head region two precavas still persist to return blood to the heart in balanced fashion (Fig. 311A). In edentates, carnivores, and primates, on the other hand, a reduction of the azygos system on the left side results in an asymmetrical shifting of most of the blood from this area to the right side for delivery to the heart, while in the head region the right precava becomes dominant in the following manner. A cross-vein, the left brachiocephalic, is laid down from the right precava diagonally across and forward to the left precava at the point where the latter is formed by the union of jugular and subclavian (Fig. 311b). The left precava between this cross-vein and the heart then degenerates except for a small part, the region of the original duct of Cuvier, which persists as the coronary sinus through which blood from the wall of the heart is emptied into the right auricle. The part of the right precava in front of the cross-vein is now known as the right brachiocephalic while the posterior part remains as the single precava which empties into the right auricle after receiving the azygos vein, the original right postcardinal (Fig. 311c).
Not only the heart but all of the larger blood vessels, arteries, veins, and lymphatics, are supplied in their outer walls (tunica adventitia) with a ramifying system of nutrient blood vessels, called vasa vasorum, or “vessels of the vessels,” for just as “shoemaker’s children must also have shoes,” so blood vessels need to be provided with a supply mechanism of their own.