The Evolution of Man, vol 2 by Ernst Haeckel (fun books to read for adults TXT) 📕

The appearance of the lungs and the atmospheric respiration connected therewith, which we first meet in the Dipneusts, is the next important step in vascular evolution. In the Dipneusts the auricle of the heart is divided by an incomplete partition into two halves. Only the right auricle now receives the venous blood from the veins of the body. The left auricle receives the arterial blood from the pulmonary veins. The two auricles have a common opening into the simple ventricle, where the two kinds of blood mix, and are driven through the arterial cone or bulb into the arterial arches. From the last arterial arches the pulmonary arteries arise (Figure 2.365 p). These force a part of the mixed blood into the lungs, the other part of it going through the aorta into the body.

From the Dipneusts upwards we now trace a progressive development of the vascular system, which ends finally with the loss of branchial respiration and a complete separation of the two halves of the circulation. In the Amphibia the partition between the two auricles is complete. In their earlier stages, as tadpoles (Figure 2.262), they have still the branchial respiration and the circulation of the fishes, and their heart contains venous blood alone. Afterwards the lungs and pulmonary vessels are developed, and henceforth the ventricle of the heart contains mixed blood. In the reptiles the ventricle and its arterial cone begin to divide into two halves by a longitudinal partition, and this partition becomes complete in the higher reptiles and birds on the one hand, and the stem-forms of the mammals on the other. Henceforth, the right half of the heart contains only venous, and the left half only arterial, blood, as we find in all birds and mammals. The right auricle receives its carbonised or venous blood from the veins of the body, and the right ventricle drives it through the pulmonary arteries into the lungs. From here the blood returns, as oxydised or arterial blood, through the pulmonary veins to the left auricle, and is forced by the left ventricle into the arteries of the body. Between the pulmonary arteries and veins is the capillary system of the small or pulmonary circulation. Between the body-arteries and veins is the capillary system of the large or body-circulation. It is only in the two highest classes of Vertebrates—the birds and mammals—that we find a complete division of the circulations. Moreover, this complete separation has been developed quite independently in the two classes, as the dissimilar formation of the aortas shows of itself. In the birds the RIGHT half of the fourth arterial arch has become the permanent arch (Figure 2.365). In the mammals this has been developed from the LEFT half of the same fourth arch (Figure 2.366).

(FIGURE 2.373. Heart and head of a dog-embryo, from the front, a fore brain, b eyes, c middle brain, d primitive lower jaw, e primitive upper jaw, f gill-arches, g right auricle, h left auricle, i left ventricle, k right ventricle. (From Bischoff.)

FIGURE 2.374. Heart of the same dog-embryo, from behind. a inosculation of the vitelline veins, b left auricle, c right auricle, d auricle, e auricular canal, f left ventricle, g right ventricle, h arterial bulb, (From Bischoff)

FIGURE 2.375. Heart of a human embryo, four weeks old; 1. front view, 2. back view, 3. opened, and upper half of the atrium removed. a apostrophe left auricle, a double apostrophe right auricle, v apostrophe left ventricle, v double apostrophe right ventricle, ao arterial bulb, c superior vena cava (cd right, cs left), s rudiment of the interventricular wall. (From Kolliker.)

FIGURE 2.376. Heart of a human embryo, six weeks old, front view. r right ventricle, t left ventricle, s furrow between ventricles, ta arterial bulb, af furrow on its surface; to right and left are the two large auricles. (From Ecker.)

FIGURE 2.377. Heart of a human embryo, eight weeks old, back view. a apostrophe left auricle, a double apostrophe right auricle, v apostrophe left ventricle, v double apostrophe right ventricle, cd apostrophe right superior vena cava, ci inferior vena cava. (From Kolliker.))

If we compare the fully-developed arterial system of the various classes of Craniotes, it shows a good deal of variety, yet it always proceeds from the same fundamental type. Its development is just the same in man as in the other mammals; in particular, the modification of the six pairs of arterial arches is the same in both (Figures 2.367 to 2.370). At first there is only a single pair of arches, which lie on the inner surface of the first pair of gill-arches. Behind this there then develop a second and third pair of arches (lying on the inner side of the second and third gill-arches, Figure 2.367). Finally, we get a fourth, fifth, and sixth pair. Of the six primitive arterial arches of the Amniotes three soon pass away (the first, second, and fifth); of the remaining three, the third gives the carotids, the fourth the aortas, and the sixth (number 5 in Figures 2.364 and 2.368) the pulmonary arteries.

The human heart also develops in just the same way as that of the other mammals (Figure 2.378). We have already seen the first rudiments of its embryology, which in the main corresponds to its phylogeny (Figures 1.201 and 1.202). We saw that the palingenetic form of the heart is a spindle-shaped thickening of the gut-fibre layer in the ventral wall of the head-gut. The structure is then hollowed out, forms a simple tube, detaches from its place of origin, and henceforth lies freely in the cardiac cavity. Presently the tube bends into the shape of an S, and turns spirally on an imaginary axis in such a way that the hind part comes to lie on the dorsal surface of the fore part. The united vitelline veins open into the posterior end. From the anterior end spring the aortic arches.

(FIGURE 2.378. Heart of the adult man, fully developed, front view, natural position. a right auricle (underneath it the right ventricle), b left auricle (under it the left ventricle), C superior vena cava, V pulmonary veins, P pulmonary artery, d Botalli’s duct, A aorta. (From Meyer.))

This first structure of the human heart, enclosing a very simple cavity, corresponds to the tunicate-heart, and is a reproduction of that of the Prochordonia, but it now divides into two, and subsequently into three, compartments; this reminds us for a time of the heart of the Cyclostomes and fishes. The spiral turning and bending of the heart increases, and at the same time two transverse constrictions appear, dividing it externally into three sections (Figures 2.371 and 2.372). The foremost section, which is turned towards the ventral side, and from which the aortic arches rise, reproduces the arterial bulb of the Selachii. The middle section is a simple ventricle, and the hindmost, the section turned towards the dorsal side, into which the vitelline veins inosculate, is a simple auricle (or atrium). The latter forms, like the simple atrium of the fish-heart, a pair of lateral dilatations, the auricles (Figure 2.371 b); and the constriction between the atrium and ventricle is called the auricular canal (Figure 2.372 ca). The heart of the human embryo is now a complete fish-heart.

(FIGURE 2.379. Transverse section of the back of the head of a chick-embryo, forty hours old. (From Kolliker.) m medulla oblongata, ph pharyngeal cavity (head-gut), h horny plate, h apostrophe thicker part of it, from which the auscultory pits afterwards develop, hp skin-fibre plate, hh cervical cavity (head-coelom or cardiocoel), hzp cardiac plate (the outermost mesodermic wall of the heart), connected by the ventral mesocardium (uhg) with the gut-fibre layer or visceral coelom-layer (dfp apostrophe), Ent entoderm, ihh inner (entodermic?) wall of the heart; the two endothelial cardiac tubes are still separated by the cenogenetic septum (s) of the Amniotes, g vessels.)

In perfect harmony with its phylogeny, the embryonic development of the human heart shows a gradual transition from the fish-heart, through the amphibian and reptile, to the mammal form, The most important point in the transition is the formation of a longitudinal partition—incomplete at first, but afterwards complete—which separates all three divisions of the heart into right (venous) and left (arterial) halves (cf. Figures 2.373 to 2.378). The atrium is separated into a right and left half, each of which absorbs the corresponding auricle; into the right auricle open the body-veins (upper and lower vena cava, Figures 2.375 c and 2.377 c); the left auricle receives the pulmonary veins. In the same way a superficial interventricular furrow is soon seen in the ventricle (Figure 2.376 s). This is the external sign of the internal partition by which the ventricle is divided into two—a right venous and left arterial ventricle. Finally a longitudinal partition is formed in the third section of the primitive fish-like heart, the arterial bulb, externally indicated by a longitudinal furrow (Figure 2.376 af). The cavity of the bulb is divided into two lateral halves, the pulmonary-artery bulb, that opens into the right ventricle, and the aorta-bulb, that opens into the left ventricle. When all the partitions are complete, the small (pulmonary) circulation is distinguished from the large (body) circulation; the motive centre of the former is the right half, and that of the latter the left half, of the heart.

The heart of all the Vertebrates belongs originally to the hyposoma of the head, and we accordingly find it in the embryo of man and all the other Amniotes right in front on the underside of the head; just as in the fishes it remains permanently in front of the gullet. It afterwards descends into the trunk, with the advance in the development of the neck and breast, and at last reaches the breast, between the two lungs. At first it lies symmetrically in the middle plane of the body, so that its long axis corresponds with that of the body. In most of the mammals it remains permanently in this position. But in the apes the axis begins to be oblique, and the apex of the heart to move towards the left side. The displacement is greatest in the anthropoid apes—chimpanzee, gorilla, and orang—which resemble man in this.

As the heart of all Vertebrates is originally, in the light of phylogeny, only a local enlargement of the middle principal vein, it is in perfect accord with the biogenetic law that its first structure in the embryo is a simple spindle-shaped tube in the ventral wall of the head-gut. A thin membrane, standing vertically in the middle plane, the mesocardium, connects the ventral wall of the head-gut with the lower head-wall. As the cardiac tube extends and detaches from the gut-wall, it divides the mesocardium into an upper (dorsal) and lower (ventral) plate (usually called the mesocardium anterius and posterius in man, Figure 2.379 uhg). The mesocardium divides two lateral cavities, Remak’s “neck-cavities” (Figure 2.379 hh). These cavities afterwards join and form the simple pericardial cavity, and are therefore called by Kolliker the “primitive pericardial cavities.”

(FIGURE 2.380. Frontal section of a human embryo, one-twelfth of an inch long in the neck, magnified forty times; “invented” by Wilhelm His. Seen from ventral side. mb mouth-fissure, surrounded by the branchial processes, ab bulbus of aorta, hm middle part of ventricle, hl left lateral part of same, ho auricle, d diaphragm, vc superior vena cava, vu umbilical vein, vo vitelline space, lb liver, lg hepatic duct.)

The double cervical cavity of the Amniotes is very interesting, both from the anatomical and the evolutionary point of view; it corresponds to a part of the hyposomites of the head of the lower Vertebrates—that part of the ventral coelom-pouches which comes next to Van Wijhe’s “visceral cavities” below. Each of the cavities still communicates freely behind with the two coelom-pouches of the trunk; and, just as these afterwards coalesce into a simple body-cavity (the ventral mesentery disappearing), we find the