Book - Vertebrate Zoology (1928) 24
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXIV The Vascular System
The vascular system is remarkably uniform in its main features in all chordates. It consists essentially of four longitudinal vessels running along the whole length of the animal. Of these, one runs under the gut in the gut-wall (subintestinal vessel) ; the other three run in the body-wall, and are the dorsal aorta and the paired cardinal veins respectively. The subintestinal vessel connects with the dorsal aorta at the anterior end of the animal by a number of paired vessels which run up round the gut on each side passing in between the gill-slits. The anterior prolongations of the dorsal aorta (which is paired in the anterior region) are the internal carotids. Farther back the dorsal aorta gives off small vessels in each septum (between the segments) to the tissues of the body-wall, and other vessels which pass down the mesentery supporting the gut to supply the gut- wall. The blood in the gut-wall is collected up into the subintestinal vessel and is led forwards again. On the way, it breaks up into capillaries again in a glandular diverticulum of the gut — the liver, and deposits much of the digested and absorbed material which it has picked up in the posterior region of the gut (intestine). In this way a hepatic portal system is formed. The blood in the body- wall makes its way to the cardinal veins, and from them it crosses the coelomic cavity between the body-wall and the gut- wall by the ductus Cuvieri (or superior vena cava), running in the transverse septum, to the subintestinal vein. This is the fundamental type on which the peripheral vessels are arranged in all chordates, and the details in the various groups can be considered under the headings, veins, heart, and arteries.
Fig. 166a. Diagrammatic representation of the heart and aortic arches. A, a fish; B, an amphibian; C, a lizard
It may be remembered that arteries are vessels leading blood away from the heart, and veins lead blood towards the heart, whatever be the kind of blood which they contain. Further, arterial blood is rich in oxygen, and venous blood poor in oxygen, whatever may be the nature of the vessel which contains it. Actually, the purest arterial blood in the body is in a vein (pulmonary), and the foulest venous blood is in an artery (also pulmonary).
The Veins
The description given above applies to the venous system of Amphioxus. In the Craniates, the presence of mesodermal kidneys (pronephros and mesonephros), lying in the track of the posterior cardinal veins, brings about the formation of a renal portal system. The anterior cardinal veins give rise to the jugulars, and in the Gnathostomes there are veins returning blood from the fins or limbs. Those from the anterior limbs are the subclavian veins which run into the ductus Cuvieri ; those from the hind limbs are the pelvic veins which run into the renal portals and into the lateral abdominal veins. The two latter veins often join in the middle line on the ventral side and give rise to the anterior abdominal vein of Ceratodus and higher forms. In the amniotes the lateral abdominal veins receive the blood from the allantois in the embryonic stages of development. Beginning in the Dipnoi, there is another connexion between the circula- tion of the body- wall and that of the gut-wall, apart from the superior venae cavae. This is the inferior vena cava. Pul- monary veins are present in Polyp terus, Dipnoi, and Tetrapods, returning blood from the lungs to the heart. In the amniotes the renal portal veins tend to diminish owing to the fact that the functional kidney of the adult is no longer a mesonephros but a metanephros, and in the birds and mammals there is no renal portal system.
The Heart
In Amphioxus there is no specialised heart in which the blood is pumped forwards, but, apart from the specialised bulbils, the whole vascular system is contractile and propels the blood along. Beginning in the Cyclostomes, there is a definite portion of the subintestinal vein in front of the liver and behind the gill-slits which is set apart as a muscular pump, and forms the heart. The veins from the liver and the ductus Cuvieri are received by a sinus venosus, which in turn leads into a thin-walled auricle. The latter passes the blood on to the thick-walled muscular ventricle, by which it is propelled into the anterior portion of the subintestinal vessel which is called the ventral aorta. The arteries are surrounded by smooth muscle, but the musculature of the heart is peculiar and unique in that it shows a number of cross-striations and its fibres branch. The openings between the various sub- divisions of the heart are guarded by valves which prevent a return flow.
In Scyllium the ventricle is produced forwards into a muscular and contractile conus, which contains several rows of valves. In front of this, the base of the ventral aorta is swollen into a non-contractile bulbus. (The walls of the conus contain heart-muscle, those of the bulbus smooth muscle.) In the higher bony fish the conus tends to disappear while the bulbus enlarges. Amia is primitive in showing a fairly large conus with three rows of valves. In the Dipnoi, the valves of the conus are well developed, and they give rise to a spiral septum which almost or quite divides the conus into two. These same forms are further very interesting in that the ventral aorta is very much shortened up into a truncus (instead of extending forwards all the way beneath the gills as in Scyllium), and also because in Ceratodus there is a beginning of the subdivision of the auricle into two, with the pulmonary veins running into the left subdivision.
In the frog, the heart is not unlike that of Ceratodus, except that the auricles are completely divided into two, and that the spiral septum in the conus and truncus is better developed, dividing a pulmonary channel (leading to the pulmonary arches) from an aortic channel (leading to the aortic and carotid arches).
In the water-breathing forms, the heart is always full of venous deoxygenated blood, while in air-breathing forms there is always a double stream of blood in the heart. One of these streams is arterial and oxygenated, and the other venous and deoxygenated. Since in the frog there is only one ventricle, and both the arterial blood from the left auricle and the venous blood from the right auricle open into it, there is a mixture in the ventricle which is sorted out into the two channels in the truncus by the spiral septum and valves. In newts, the septum between the auricles tends to break down, as does the septum in the truncus. In the embryonic stages of amniotes the septum between the auricles remains incom- plete also, until the time of hatching or birth, in connexion with functional details of the embryonic circulation.
The hearts in the amniotes fall into two classes, neither of which can be derived from the other, and which must have been separately evolved from the amphibian condition. The conus is reduced and incorporated in the wall of the ventricle, but while in one group which may be called Sauropsidan the truncus is split right down to the ventricle into three channels, in the other or Theropsidan group it is split into only two channels.
The three channels in the Sauropsida are the pulmonary, the right systemic, and the left systemic. The two latter cross over one another so that the right systemic springs from the left side of the ventricle, while the left systemic arises with the pulmonary from the right side of the ventricle. In the lizards, snakes, tortoises, and Sphenodon, the ventricle is still single, although there is a septum which divides it incompletely. The left auricle, as always, contains the arterial blood, most of which goes into the right systemic arch. In the crocodile, the interventricular septum is complete, but it is formed in such a way that while the right systemic arch gets all the arterial blood from the left auricle, the left systemic arch together with the pulmonary, gets only venous blood from the right auricle. There is a small foramen (of Panizza) between the right and left systemic arches which allows a little inter- change of blood. The condition in the bird is like that of the crocodile except that the left systemic arch has been abolished altogether, which is not surprising, seeing that it could only distribute blood which is almost purely venous. In the bird, therefore, with its four-chambered heart, there is no mixture of arterial with venous blood ; all the venous blood in the heart goes to the lungs and only to the lungs. In reptiles and birds, the carotids arise from the right systemic arch.
The two channels of the truncus in the Theropsida are the pulmonary and the single systemic aorta. These forms include the mammals, and the Theromorph reptiles, although the latter (fossils) are obviously only known from their skeleton. The heart is four-chambered, and the ventricle is completely divided into two, so that all the venous blood from the right auricle goes into the pulmonary arch, and all the arterial blood from the left auricle into the systemic aorta, and there is no mixture. It so happens that the aortic arch of the right side does not persist, and only the left one remains, but it is of the utmost importance to realise that the reason why there is a single systemic arch in the bird is totally different from that which is responsible for the single arch in the mammal. The structure of the heart in the amniotes shows that the reptiles contain two main lines of evolution (besides other less important lines), the one culminating in the birds and the other in the mammals. The sinus venosus disappears in the highest forms, birds and mammals, and is represented by the so-called sino-auricular node. This structure is of great functional importance, for it acts as the pace-maker to the heart. It is here that the contraction originates, which contraction then becomes taken up by the other parts of the heart, and constitutes its " beat." In birds and mammals the superior and inferior caval veins open direct into the right auricle. The sino-auricular valves give rise in the mammals to the Eustachian and Thebesian valves.
The Arteries
In the fish typically, each of the visceral arches has an afferent branchial artery leading from the ventral aorta to the gills, and an efferent branchial artery connecting the gills to the lateral dorsal aorta. The vessels in the mandi- bular arch become reduced. The general arrangement of these vessels is necessitated by the presence of the visceral clefts, which make it impossible for the vessels to reach the dorsal side of the gut from the ventral side except by passing in the visceral arches between the clefts. Since gill-slits or pouches are present in the embryos of all chordates, the same reason accounts for the arrangement of the arterial arches in the higher forms. In the air-breathing vertebrates, the gills are reduced and there is a continuous vessel in each visceral arch running from the truncus arteriosus (ventral aorta) to the lateral dorsal aorta. In Salamandra all the vessels in the 3rd to 6th visceral arches persist. The 3rd becomes the carotid, the 4th and 5th become systemics, and the 6th is the pul- monary. All these arterial arches place the truncus in com- munication with the lateral dorsal aorta. The lateral dorsal aortas are, however, interrupted between the dorsal ends of the 3rd and the 4th arterial arches ; i.e. there is no ductus caroticus. The conditions in Triton are similar except that the 5th arterial arch has completely disappeared. In Lacerta (as in all higher forms) the 3rd arch persists as the carotid, the 4th as the systemic, and the 6th as the pulmonary. In Lacerta, the connexion between the dorsal ends of the arteries of the 3rd and 4th arches persists, forming the ductus caroticus. The lateral dorsal aorta is here accordingly uninterrupted. The ductus caroticus is absent in the adult of higher forms. The connexion between the pulmonary arch and the lateral dorsal aorta is the ductus arteriosus. This connexion is important in the embryonic stages of Amniotes. It enables the blood from the right side of the ventricle (or the right ventricle, if it is separated off) to reach the lateral dorsal aorta through the pulmonary arteries, instead of going to the lungs. At these early stages of development the lungs are not yet open. In the adult amniote, the ductus arteriosus usually degenerates into a ligament, as, for example, in the mammal (on the left side), or disappears. It persists, however, in some turtles, and their case is interesting, for they are in the habit of diving, and during the submerged period the lungs are not working. The blood in the pulmonary artery can then escape into the general circulation without going through the lungs. The ductus arteriosus is also called the ductus Botalli.
In the frog, there is neither ductus caroticus nor ductus arteriosus in the adult.
In the Sauropsidan reptiles, the right and left systemic arteries of the 4th arch are separate right down to the base of the truncus. The left one of these arches is absent in the bird. The subclavian arteries come off from the right systemic arch in lizards (dorsal type of subclavian) ; in Chelonia, crocodiles, and birds, the subclavian arteries are given off from the carotids (ventral type).
In the mammal, the right and left arteries of the 4th (systemic) arch differ from those of the Sauropsidan reptiles in that the aorta is undivided, instead of being split to its base. The artery on the right side does not reach round to the dorsal aorta ; it is given the name of innominate artery, and it leads to the right carotid and subclavian arteries. That on the left side forms the so-called aorta, gives off the left carotid and subclavian arteries, and continues back as the dorsal aorta. It is connected with the pulmonary arch by the ductus arteriosus as already mentioned.
The internal carotid arteries are the anterior prolongations of the lateral dorsal aortae, and they enter the skull by passing up between the trabecular, close to the pituitary body. The external carotids are the anterior prolongations of the ventral aorta, on each side of the thyroid.
The proximal ends of the arteries and veins are joined at the heart. The distal ends of the arteries are connected with those of the veins by the capillaries, so that the whole vascular system is a closed one. When a vein starts from capillaries and breaks down into other capillaries again before reaching the heart, it is known as a portal vein. The hepatic portal vein occurs in all chordates, the renal portal appears in the Cyclostomes and disappears in the amniotes.
The blood of Amphioxus is colourless, but in all higher forms, haemoglobin, a respiratory pigment, is present in corpuscles, which become known as " red blood-corpuscles. " In the adult mammal, these corpuscles are peculiar in being non-nucleated. The white corpuscles of the blood play an important part in the defence of the organism against invasion by foreign bodies. In the embryo, the blood arises from blood- islands, between the mesoderm and the endoderm in the region of the yolk. In the adult, blood-corpuscles are formed in the marrow of the bones, and in the lymphatic organs. The blood is under pressure in the arteries and capillaries, owing to the contraction of the smooth muscle surrounding the former and of the Rouget-cells which compose the walls of the latter.
Lymphatics
Attention may now be turned to the lym- phatic system. In addition to the blood-vessels, the body contains a system of vessels, channels, and spaces in which lymph circulates, forming the lymphatic system. It is in communication with the ccelomic cavity. Lymph is blood- plasma and white corpuscles which exude from the capillaries and bathe all the tissues of the body, supplying them with nutritive products. From the tissues, the lymph (which may thus be regarded as blood minus the red blood-corpuscles) is gathered up into thin-walled channels, called the lymphatics. These start from blind ends and eventually join the veins, in particular the subclavians, the left of which receives the main lymphatic trunk which is known as the thoracic duct. In the amphibia the space between the skin and the muscles of the body-wall is occupied by lymph, and in certain regions " lymph- hearts " are present, with muscular walls, which propel the lymph along. These lymph-hearts are lacking in mammals. Lymphatic vessels are present in the wall of the intestine, and are known as lacteals, for they absorb the fatty products of digestion, and the milk-like emulsion which they contain gives them a white appearance. Here and there along the lym- phatics, lymph glands are formed. To these belong the spleen (which first appears in the Selachii), the tonsils (derived from the 2nd pair of visceral pouches), and Peyer's patches along the intestine in the mammals.
Literature
Goodrich, E. S. Vertebrate Craniate, Cyclostomes and Fishes. Black, London, 1909.
Goodrich, E. S. On the Classification of the Reptilia. Proceedings of the Royal Society, Ser. B, vol. 89. 1916.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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