Book - Vertebrate Zoology (1928) 25
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXV The Respiratory System
All chordates have a closed vascular system and haemoglobin as a convenient transporter of oxygen. Their respiratory systems involve structures in which blood-vessels are brought into close contact with the surrounding medium (water or air) with as little intervening tissue and as great an exposed surface as possible. The former requirement is met by the very thin nature of the epithelium covering the blood-vessels, and the latter by reducing the size of the blood-vessels to capillaries, which therefore have a large surface compared with their volume.
The respiration of embryos within their membranes is effected by various means, such as the circulation of the yolk-sac or of the allantois, as has been described in connexion with the development of the frog, chick, and rabbit.
After the embryonic stage has been passed, chordates breathe either by gills, or by gills and lungs (sometimes assisted by the skin), or by lungs alone.
Gills are groups of capillaries in the walls of the gill-slits, through which water passes out from the pharynx. In Amphioxus the current of water is caused by the action of the cilia on the under side of the oral^ hood and in the gill-slits themselves. Fish breathe in the following manner : the gill- slits are shut and the floor of the mouth is lowered, which causes water to enter the mouth. The mouth is then closed, its floor is raised, and the water escapes through the gill-slits. When Cyclostomes are feeding, they are firmly attached to their prey by their mouth and the sucker surrounding it. They cannot therefore take in water through the mouth, and the gill-pouches are modified into sacs which pump water in and out again. In the larvae of some fish (Polyp terus, Lepidosiren), and in those of amphibia, external gills may be developed in the form of tuft-like structures projecting out from the body into the water, and which enable the blood to be oxygenated before the gill-slits are pierced. The larval amphibia afterwards develop ordinary gills on the outer faces of the gill-arches, and their respiration is like that of the fish. In all these cases the respiratory movements are brought about by means of the contraction of visceral muscles, innervated by dorsal cranial nerve-roots, and controlled by a centre in the medulla oblongata.
The first visceral cleft or spiracle is open in the Selachii, but it is closed in all higher forms with the exception of Polypterus and Acipenser (the sturgeon). There may be a spiracular gill, which is called a pseudobranch because its capillaries receive blood which has already been oxygenated in the next posterior (true) gill. In the Tetrapods the cavity of the spiracular cleft gives rise to the tympanic cavity and Eustachian tube.
The rays are Selachii adapted for living on the sea-bottom, and they are of a flattened shape, with the gill-slits on the under side. The spiracle is on the upper side, and serves to admit water into the pharynx. In the Selachii, the gill-slits are uncovered, but in the bony fish (Dipnoi and Teleostomes) they are covered over and protected by an operculum. An analogous operculum develops in the larva of the frog, and it may be remembered that in Amphioxus the gill-slits are protected by being enclosed in the atrial cavity.
The gill-sacs of Petromyzon all open independently to the exterior, whereas those of Myxine have a single joint opening on each side.
The number of gill-slits in Amphioxus is large (up to 180). In Selachii, not counting the spiracle, it is five, except in Heptanchus which has seven, and Hexanchus and Pliotrema which have six. Five is also the number in bony fish. It is important to remember that gill-slits or pouches are present in early stages of development of all chordates up to and including mammals, and that they play a part in the disposition of the arterial arches although they cease to function as respiratory organs.
The adult Amphibia (or most of them, i.e. those which have not lost the lungs) and all higher vertebrates breathe by lungs. (The use of the skin as a breathing organ in Amphibia is made possible by the fact that their skin is moist and uncovered.)
Lungs are also present in some fish. In Polyp terus, there is a trachea leading out from the ventral side of the oesophagus, and forking into two lungs. The cavity of these lungs is divided into small spaces or " cells," which has the result of increasing the internal surface. Such lungs are called cellular, and they are supplied with blood by pulmonary arteries, i.e. branches from the last (6th) pair of branchial arterial arches. From them, blood returns (to near the sinus venosus) by paired pulmonary veins. In the Dipnoi, there are paired lungs in Protopterus and Lepidosiren, but a single one only in Ceratodus. Their relations are similar to those of Polyp terus, except that the lungs, together with the pulmonary arteries and veins, have been displaced to a dorsal position by passing round the right side of the oesophagus. In Ceratodus the pulmonary veins open into the left side of the auricle. Lungs were almost certainly present in the Osteolepidoti. These animals lived or live in fresh water in which the oxygen- content is low (owing to desiccation and accumulation of decomposing organic debris), and branchial respiration is supplemented by the intake of bubbles of air through the mouth. Indeed, Protopterus is able to withstand periods of drought when the swamps in which it lives dry up, by burying itself in the mud and breathing by its lungs. The lungs of higher vertebrates are easily derived from those of the fish just described. It is possible that the lungs respresent a pair of gill-pouches behind the remainder, and which ceased to open to the exterior. They are formed from the endoderm and communicate with the alimentary canal, and they preserve their blood-supply from the vessel of the last branchial arch.
In the higher bony fish, the lung is single and modified.
In the primitive form Amia, it is still supplied with blood from the last branchial artery and its walls are cellular, but in all the rest it derives blood from the coeliac artery and dorsal aorta, and its walls are not adapted for the diffusion of gases through them, except in a restricted vascular area. In some forms it remains connected with the alimentary canal by an open tube, but in others it is completely shut off (in the adult condition). In these higher bony fish, the lung no longer functions as a respiratory organ, but it has become a hydro- static organ. The quantity of gas which it contains is regulated by the vascular area just referred to (where oxygen may be passed from the blood into it or vice versa), and the fish is able to adapt its specific gravity to that of the depth of the water at which it is swimming. It is therefore able to maintain its depth without muscular effort. In these forms it is no longer a lung, but an air-bladder or swim-bladder. In some Teleosts, such as the catfish (Amiurus), the swim-bladder enters into relations with the auditory vesicle, and is connected with it by a chain of small bones called the Weberian ossicles, which are derived from the first three vertebrae. In some other Teleosts, the swim-bladder disappears in the adult, and these are often found to be bottom-living forms, which live at a more or less constant depth.
Strange as it may seem, therefore, it is probable that the lungs were evolved while the vertebrates were still in the water, and that they gave rise to the swim-bladder by specialisation.
It is now necessary to turn to the relations which the olfactory organs bear to the respiratory system. In the Selachii and the higher bony fish, the nasal sacs have no connexion with the mouth, but this is not the case in the most primitive bony fish. In Osteolepis and in the Dipnoi there are external nostrils on the snout, and they lead to internal nostrils which open into the mouth-cavity. This condition is also present in all the Tetrapods. In these forms, therefore, the olfactory organs are subservient to the respiratory system in that they enable the respiratory medium (water or air) to enter the mouth- cavity without having to pass through the mouth itself. It may be remembered that in Petromyzon the nostril is single and confluent with the opening of the hypophysial sac. The same is true of Myxine, but here the hypophysial sac opens into the alimentary canal. This con- nexion between nose and gut is, however, quite different from that of the other forms just mentioned, and was independently acquired.
The amphibia when adult breathe air into their lungs, but the mechanism for doing so is similar to that which the fish use for breathing with their gills. The floor of the mouth is lowered and air is taken into the mouth cavity. The mouth and nostrils are then closed, and the floor of the mouth raised, which forces the air down the throat and larynx into the lungs.
The method of respiration in the amniotes is more efficient. The volume of the lungs is increased by the expansion of the thoracic box, and this is accomplished by movements of the ribs (assisted in the mammals by movements of the diaphragm). The muscles concerned in these movements are somatic and innervated by ventral nerve-roots of the neck and thorax. The tortoises, whose ribs are, of course, fixed to the carapace which surrounds them, replenish the air in their lungs by movements of the neck, arms, and legs.
The lungs of Polypterus, Dipnoi, and amphibia are more or less hollow sacs. In reptiles the internal surface of the lungs are increased by foldings of the walls, with the result that the lungs can no longer be described as simple hollow sacs. In birds and mammals, this process has been carried still further, and the lungs are spongy masses of tissue penetrated by innumerable small air-spaces. In mammals, the internal surface-area of the lungs may be thirty times that of the external surface of the body.
The lungs of the chamaeleon are of interest in that they are produced into a number of blind diverticula or air-sacs. These air-sacs reach their highest degree of development in the birds, in which they may occupy a large volume. Air is led into the air-sacs from the bronchi passing straight through the lungs, and it then passes back into the lungs where it oxygenates the blood, and out again through the trachea. The efficiency of this mechanism lies in the fact that there is a through- draught right through the lungs. All the air can be renewed, whereas in other forms, the lungs are blind sacs and there is always a certain amount of stale residual air at the bottom of them which cannot be renewed. The efficiency of the respiratory system has played a large part in the evolution of the birds, which require a high rate of metabolism in order to perform the very arduous muscular exertion of maintaining the body in the air during flight.
Attention may now be turned to two modifications which may occur in connexion with the respiratory system. The first concerns the formation of the false palate. This structure is a secondary roof to the mouth, closing over the original internal nostrils, and enclosing the nasal passage as far back as the secondary choana. The secondary choana is opposite the glottis (the opening through which the pharynx com- municates with the larynx and trachea, and so with the lungs), and the whole structure is an adaptation enabling the animal to breathe and yet have its mouth full of food or water at the same time. It is especially developed in aquatic forms such as the crocodile and the whale, but it is character- istic of the higher Theromorph reptiles and mammals in general. In the whales the glottis can be pushed right up into the secondary choana, thus making a closed communication between the external nostrils (above the surface of the water) and the lungs, without running the risk of water entering the latter from the mouth. In the higher vertebrates, and especially those which frequent deep waters, the windpipe or trachea is prevented from collapsing by rings of cartilage or bone.
The fact that respiration in terrestrial vertebrates involves the pumping of air in and out of the body, has been made use of in connexion with the production of sound. Bands of connective tissue stretch across the cavity of the larynx, and can be thrown into vibration by the passage of the air. These bands are the vocal cords. In the male frog there are vocal sacs at the corners of the mouth, and these become distended with air when the animal " croaks " and act as resonators.
The larynx and its vocal cords are the organ of voice- production in the mammal, and the pitch of the sounds can be controlled by the tension of the cords and the laryngeal muscles. The false palate acts as a resonator. In the birds there is a special organ called the syrinx situated at the fork where the trachea divides into the two bronchi, and it is to the vibrations of this that the song of birds is due.
It is interesting to note that the power of producing vocal sounds has evolved parallel with the capacity for appreciating them, or in other words, the differentiation of the cochlear part of the ear.
Literature
Goodrich, E. S. Vertebrata Craniata. Cyclostomes and Fishes. Black, London, 1909.
Oppel, A. Atmungsapparat : Lehrbuch der Vergleichenden Mikrosko- pischen Anatomie der Wirbeltiere. Part 6. Fischer, Jena, 1905.
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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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