34 



VERTEBRATE RESPIRATION 



Dorsal aorta 



Oval 

 Sphincter 



Hepatic .-fiv; 



portal vein Rete 



mirabile Gas 

 gland 



(b) 



Gas 

 gland 



200 Ats. 

 O2 tension 



Fig. 10. 



Diagrams to show the structure of a physocHstous swim bladder, 

 (a) General organisation and blood supply, (b) Transverse section 

 through the rete mirabile to show the close relationship between 

 afferent and efferent capillaries, (c) Diagram illustrating the role of 

 the hair-pin counter-current in maintaining the very large difference 

 in O2 tension between the contents of the bladder and the blood 

 supply in a deep-sea fish. The arrows indicate the diffusion of oxygen 

 across the rete. (b and c after Scholander, 1958.) 



pletely understood process. It is associated, however, with a 

 remarkable structure of the capillaries in the gas gland. The 

 afferent vessel divides into a large number of parallel running 

 branches which come near to the surface of the secretory 

 epithelium and then double back on themselves in a sort of hair- 

 pin bend and on their return become very closely apposed to the 

 afferent capillaries. A cross section (fig. 10b) of this whole 

 structure, the rete mirabile, gives the impression of a draughts 

 board, the black and white squares of which represent the 

 afferent and efferent vessels respectively. It is evident that such 

 a structure will greatly facilitate the exchange of gases between 



