Factors Influencing Submergence and the Heart Rate in the Frog.

INTRODUCTION In view of the intermediate position held by the amphibians between air-breathing and water-breathing vertebrates, surprisingly little is known of the external or internal factors regulating respiratory exchange and related circulatory phenomena in these animals. Spurway & Haldane (1953) have suggested that the breathing behaviour in the Amphibia represents the complete expression of a pattern which becomes truncated in the air-breathing descendants and regained in diving birds, reptiles and mammals. Whether this controversial view is accepted or not, there can be no doubt that a study of respiratory exchange in the Amphibia must give some insight into the development of an air-breathing from the earlier water-breathing system. Before examining the detailed neural pathways involved in respiratory regulation, however, it is important to know what environmental factors govern gas exchange in air and water and so make it possible for the animal to submerge or to breathe in air. It has been known for some time that submersion produces respiratory and circulatory changes of approximately the same type in those reptiles, birds and mammals which are habitual and well-adapted divers (Andersen, 1961, 1963; Irving, Scholander & Grinnell, 1941; Johansen, 1959; Scholander, 1940; Scholander, Irving & Grinnell, 1942) and to some extent in pearl divers who are not so well adapted (Scholander, Hammel, LeMessurier, Hemingsen & Garey, 1962). These changes consist of apnoea and a slowing of the heart (bradycardia) together with a curtailment of blood flow in which the muscles, periphery, and most of the visceral organs are depleted of blood. As far as has been established the circulatory changes are entirely reflex, whereas the apnoea may be reflex or may contain 'voluntary' components. Certainly many factors are known to affect the inhibition of the rhythmic respiratory centre and in man at least, as Spurway & Haldane (1953) point out, excitation always overcomes the inhibitory processes for the lungs of drowned men always contain water. Whether excitation tends to become dominant and so perhaps be the significant factor in causing the other diving vertebrates to break surface is not clear. Although a certain amount is known about the mechanics of lung ventilation in Amphibia (Krogh, 1941; de Marneffe-Foulon, 1962) and about the relative importance of skin and lungs as the exchanging surface (Krogh, 1904; Dolk & Postma, 1927), there is very little information for these semi-aquatic animals which can be compared with that available for the less well-adapted diving vertebrates. During submersion the requirements of an amphibious vertebrate would be different in some respects from those of the diving but purely air-breathing types. Rhythmic activity in the respiratory