358 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



-land. The actual process of secretion is carried out not by 

 the entire lining of the organ but by patches of special glandular epithelium 

 known as red bodies or red glands from their highly vascular character. 

 These reach their highest development in the physoclistic fishes. In 

 tlii-se the columnar glandular epithelium of the red gland, the cells of 

 which secrete bubbles of gas like the gas vacuoles of Protozoa,, dips 

 down into deep crypts between which pass up loops of blood-vessel. 

 These show a very remarkable arrangement, both the afferent and the 

 efferent vessel of each loop being broken up for a considerable distance 

 into a number of fine parallel channels. Such subdivision of blood- 

 vessels into a number of channels appears to be a safeguard against 

 occlusion of the vessel through great pressure. Well-known examples 

 of the same phenomena are seen in the blood-vessels of Whales which 

 break up to form what were called by the older anatomists retia mirabilia. 

 It is obvious that the wall of the air-bladder in a fish will be subjected 

 to great pressure if it swims for some distance upwards before the 

 expansion of the contents of the air-bladder is corrected. The gas- 

 glands of the fish are under the control of the nervous system and one 

 of the first striking bits of evidence indicating that their mode of action 

 is analogous to that of ordinary glands is the fact that interference with 

 their nerve supply produces exactly the same results as it does in the 

 case of such typical glands as those of the stomach in one of the higher 

 vertebrates severing the branches of the vagus which supply them 

 bringing about a cessation of their activity. 



Diminution in the amount of gas is brought about by a process of 

 absorption ; and in the more highly developed air-bladders the absorptive 

 power is concentrated in a special portion of the lining of the air-bladder 

 termed the oval. This is a highly vascular area covered by epithelium 

 which appears to have the power of absorbing gas from the cavity of 

 tin- air-bladder and of passing it away in solution into the blood. The 

 regulation of its effect is brought about by a muscular arrangement 

 resembling the iris - diaphragm of a microscope which more or less 

 completely closes over it so as to separate its surface from the gaseous 

 contents of the organ. 



It remains to inquire how the mechanism of the air-bladder is awakened 

 into activity. The lining of the organ contains numerous sensory-cells 

 and it is probably messages from these to the central nervous system 

 which arouse a reflex activity, either secretive or absorptive as may be 

 required. 



As regards the region of the alimentary canal behind the pharynx 



