SCOTT, STUDY OF CHANGES IN MUSTELUS CANIS 51 



blood takes place by diffusion through the gills and that the gill mem- 

 branes become permeable to them. 



Eegulation" of the Osmotic Pressure of the Blood of Mustelus 



A constant osmotic pressure of the blood is regarded as necessary for 

 the normal activities of cells and tissues of the higher forms. The kid- 

 neys are recognized as being primarily concerned in maintaining this 

 constant pressure. Their activity in this connection will be considered 

 later. In addition to the kidneys, it has been pointed out by Buglia ('09) 

 that the tissues take part in this regulation. Buglia found that injec- 

 tions of hypotonic salt solutions into the circulation of a dog produced 

 little effect on the molecular concentration of the blood. He concluded 

 that the excessive water disappeared with astonishing rapidity from the 

 blood plasma by entering the cells or tissues. In this way the normal 

 osmotic pressure of the blood was maintained. Japelli (^06) found after 

 intravenous injections of hypotonic solutions of sodium chloride into the 

 circulation of the dog, that the muscles took up water from the blood, 

 thus exerting a regulative action on the osmotic pressure of the blood. 

 When Mustelus is immersed in fresh water, its tissues are bathed by di- 

 luted blood. Is there any evidence of an attempt on the part of the tis- 

 sues to maintain the normal osmotic pressure of the blood by taking up 

 water from the hypotonic blood which bathes them? Various organs, 

 namely, the brain, heart, kidney, spleen and muscle, were removed from 

 several dog-fishes which had been in sea-water. The same organs were 

 removed from other fishes that had been immersed in fresh water until 

 near death. Each organ was placed on filter paper, the heart and brain 

 being cut open, and the other organs being cut into small pieces. All 

 free fluids were removed with filter paper. Each organ was then weighed 

 and put at first into a hot-air bath at 100° C. for a time and then into a 

 dessicator over sulphuric acid. A partial vacuum was made by withdraw- 

 ing air by means of a filter pump. When the organs were dried to con- 

 stant weight, the percentage of water in each case was calculated. The 

 results of this experiment are shown in Table XX. 



