Chap, v.] RETIA MIRABILIA. 



209 



the escape of the blood corpuscles (diapcedesis), or, in 

 general terms, the relation of the moving nutrient and 

 respiratory currents to the surrounding cells is im- 

 proved by such a slowing. This will explain the 

 presence of a rete mirabile in the reduced anterior gill 

 (pseudobranch) of many fishes, or in the walls of their 

 air bladders, for by this means the exchange of oxygen 

 is the more successfully effected ; a similar reason may 

 be given for the great development of retia in the 

 thoracic and costal regions of the Cetacea, or the very 

 general distribution of small plexuses in the glomeruli 

 of the kidney. (See page 258.) 



On the other hand, the mere mechanical advantage 



O 



of the slowing of the blood current must be of impor- 

 tance in forms such as the Ruminantia, in which the 

 head is often for a long period at a lower level than 

 that of the carotids, and by their presence the dangers 

 of a flow of blood to the head may be averted ; the same 

 kind of explanation may, no doubt, be applied to the 

 retia in the course of the abdominal vessels, the pres- 

 sure on which must vary greatly with the extent of 

 distension of the walls of the intestinal tract. 



It is more difficult to explain the function of the 

 retia mirabilia in the eyes of Fishes and Birds; or in the 

 course of various vessels in monotremes and edentates. 

 In the latter case the lowly position of these Mammals 

 may, perhaps, be of significance. 



The earlier division of Vertebrates into hot-blooded 

 and cold-blooded forms disappeared before the pro- 

 gress of morphological discovery ; in addition to the 

 evidence that we now have as to the relationship of 

 birds to reptiles, we have further to support us the 

 now generally recognised difference between homoplasy 

 and homogeny ; in other words, it is now clear to us 

 that, with given structural arrangements, two forms, 

 not closely allied, may, under similar external con- 

 ditions, acquire a physiological resemblance ; and we 

 o16 



