EFFECTOR MECHANISMS 403 



for 2 and 3 atm for C0 2 . Since the partial pressures of gases in the blood 

 would be only fractions of an atmosphere, these figures reveal the magni- 

 tude of the pressure gradient against which 2 and C0 2 must be secreted 

 (36tf, 60a). 



The swim-bladder gas contains different proportions of the chemically 

 inert gases, argon, neon, etc., from those present in the mixture breathed 

 by the fish. Relative to nitrogen the proportions of the very soluble argon 

 are increased, and the less soluble neon and helium, decreased. It would 

 seem that oxygen is secreted into the air-bladder as small bubbles, into 

 which the inert gases diffuse ; these bubbles carry the inert gases with them 

 into the air-bladder (128a). 



A secretion of gas into the swim-bladder can be detected if a fish is 

 subjected to an increase in hydrostatic pressure, or if gas is removed from 

 the bladder by puncture. Conversely, over-inflation causes absorption of 

 gases. After stimulation of the gas-bladder of Fundulus heteroclitus by 

 deflation and over-inflation, the gas content of the bladder showed the 

 following maximal ranges (the first figure refers to a fish recovering from 

 deflation, the second to one recovering from over-inflation): 2 , 82, 

 0%;CO 2 , 23-2, 0%; N 2 , 85, 100%. 



The mechanism of gas secretion has aroused much interest. It has been 

 suggested that the secretion of oxygen depends on an increase of acidity 

 in the blood as it passes through the gas-gland. When the blood becomes 

 more acid the affinity of haemoglobin for oxygen is diminished (Bohr effect) ; 

 moreover the oxygen capacity may be diminished in the presence of acid. 

 It has been shown, however, that the acidified blood of certain deep-sea 

 teleosts is saturated at oxygen tensions lower than those existing in the 

 swim-bladder. It is unlikely, therefore, that a mechanism based on the 

 Bohr effect exists for splitting off oxygen from the blood. The very close 

 association of long afferent and efferent capillaries in the rete mirabile, 

 arranged on the counter-current principle, allows maximal exchange of 

 gases by diffusion between these two sets of vessels. The effluent blood, in 

 consequence, may be thoroughly drained of oxygen. It is beginning to 

 appear as if the glandular epithelium of the gas-gland is responsible for 

 actively taking up and secreting this oxygen into the bladder (114). 



Inflation and deflation of the swim-bladder are controlled by reflexes 

 initiated by stimulation of exteroceptors (eyes and labyrinth). Efferent 

 fibres to the swim-bladder pass along autonomic nerves (pneumogastric 

 and sympathetic), but many aspects of control still have to be worked out. 

 When the bladder is actively secreting, the gas-gland becomes dilated with 

 blood and the secretory mucosa relaxes and the resorbent mucosa con- 

 tracts; opposite changes occur in the mucosa when gas is being resorbed 

 and the bladder is undergoing deflation. After vagotomy the secretory 

 mucosa relaxes and the resorbent mucosa contracts, vessels of the gas- 

 gland become constricted and gaseous secretion ceases. Stimulation of the 

 intestinal vagus nerve causes reciprocal effects. Section of the sympathetic 

 nerve supply causes only a slight rise in the 2 content of the bladder. The 



