OSMOTIC PRESSURE 165 



the cells must have altered their own osmotic pressure to compensate for 

 the change in that of the body fluid. 



The same behaviour is shown by the lower fish, the Selachians. But, as 

 we ascend the scale of evolution, we find that the blood is maintained at a 

 nearly constant osmotic pressure by regulative mechanisms. The following 

 values from Bottazzi's article (1908) apply to marine organisms: 



Selachian fish - - A =2 '26 



Teleostean fish - ,, 1 -04-0 "76 



Reptile, turtle - 0'61 



Mammal, whale - 0< 65-0'7 



In the Teleostean fish we find the regulative mechanism in process of develop- 

 ment. Dakin (1908) found that the depression of the freezing point of the sea 

 water at Kiel was 1'09, while at Heligoland it had risen to 1'9 ; correspondingly, 

 that of the blood of the ray (a Selachian) rose in agreement. That of the plaice 

 (a Teleostean), on the contrary, had risen from 0- 66 to 0'8 only, i.e., by 20 per 

 cent., while that of the water had risen by 74 per cent. The cod is still more 

 independent of the medium ; when the A of the sea water rose from l-2 to 1'9, 

 that of the cod only rose from 0'73 to 0> 757, by 3'9 per cent. only. 



We notice that, as the power of osmotic regulation becomes more manifest in 

 the animal scale, the A of the blood tends to be fixed at about 0< 6, which is the 

 value of that of the higher land vertebrates. 



The advantage of a fixed osmotic pressure will be clear if we remember that it 

 is due, almost entirely, to salts. Colloidal systems, such as protoplasm is, are 

 especially sensitive to electrolytes, as we saw in Chapter IV., and fine adjust- 

 ments of such processes are the more perfect, the greater the constancy of the 

 electrolyte concentration of the medium in which they take place. 



The regulation of the osmotic pressure of the blood to a constant value is shown in an 

 interesting way by some observations of Cohnheim (1912, 1). Sweat contains a considerable 

 amount of salts, having a A of about 0'5, according to Tarugi and Tomasinelli (1908). Now 

 Cohnheim found that he lost a certain considerable weight in this manner by performing 

 a mountain ascent in hot weather. This weight could only permanently be replaced if he 

 drank water containing sufficient salts to replace those lost. Distilled water was rapidly lost 

 again through skin and kidneys. 



EFFECTS OF CELL METABOLISM 



The chemical changes associated with those cell activities which result in the 

 setting free of energy usually consist in the splitting up of large complex molecules 

 into a greater number of smaller ones, such, for example, as the oxidation of one 

 molecule of glucose into six molecules of carbon dioxide and six molecules of water. 

 The osmotic pressure of a solution being in proportion to its molar concentration, 

 it is clear that, neglecting the water, the osmotic pressure of a glucose solution 

 would be raised to six times its value. The bearing of this fact on the formation 

 of lymph in active organs will be seen presently. 



The mere addition of carbon dioxide to blood raises the osmotic pressure of the latter 

 considerably more than the molar concentration of the added substance would account for. 

 Kovacs (1902) states that addition of carbon dioxide to rabbit's blood raises the A in ten minutes 

 from 0'6to 0'72. The effect is due to a complex reaction with the salts of blood, which will 

 be discussed in the next chapter. 



LYMPH PRODUCTION AND ABSORPTION FROM TISSUE SPACES 



It is rarely that the blood vessels lie in immediate contact with the tissue 

 cells, whose food requirements the blood supplies and the products of whose 

 metabolic changes it carries away. There intervenes a space, of varying dimen- 

 sions, containing a fluid, the lymph, whose composition is very similar to that of 

 the blood, minus its red corpuscles, although usually containing less protein. This 

 lymph is being produced continually at a more or less rapid rate by transudation 

 from the blood vessels, and carried back to the blood by means of the lymphatic 

 vessels. Filtration is one of the factors concerned in its production, since the 

 intra vascular pressure is greater than that in the tissue spaces ; but Starling 



