214 



RESPIRATION 



On microscopic section of the wall of the swim bladder we find 

 that at most parts it is lined by flattened epithelial cells similar in 

 outward appearance to those covering the oval. At certain parts, 

 however, this flattened epithelium passes into a layer consisting of 

 cubical or columnar epithelial cells, and forming the so-called 

 "epithelial body" (Figures 60, 61), or else a convoluted layer of 

 columnar epithelium (Figure 62). In the glandular structure 

 ducts containing gas may be seen (Figures 60 and 61) in certain 

 species of fishes, and the gland is evidently an oxygen-secreting 

 gland. The true glandular structure was one of Johannes Miiller's 

 many discoveries about glands. 



ti^t.; 



R.M. 



G.Z. 



Figiire 63. 

 Diagram of circulation in rete mirabile of eel. R.M. rete mirabile. G.E. gland 

 epithelium. Arterioles and arterial capillaries continuous lines. Venules and venous 

 capillaries interrupted lines (Woodland). 



Beneath the glandular structure is a mass of red blood vessels, 

 forming a structure which attracted the attention of anatomists 

 hundreds of years ago*^ and came to be known as a rete mirabile. 

 The arrangement of the blood vessels in this "red body" was re- 

 cently studied by Woodland,^ who established the fact that the 

 rete mirabile is an arrangement in which the arterioles passing 

 to the gland break up into capillaries which come into intimate 

 contact with corresponding venous capillaries from the venules 

 coming from it (Figure 63). What is the significance of this? 

 The arrangement reminds us of that in a regenerating furnace, 

 where the heat carried away in the waste gases is utilized to heat 



Redi, Observations sur les animaux vtvans contenus dans les animaux vivans. 

 Florence, 1684. 



'Woodland, Proc. Zool. Soc. of London, p. 183, 191 1. 



