122 PRINCIPLES OF GENERAL PHYSIOLOGY 



lipoid solubility. In fact, the mineral acids were far less active than the fatty 

 acids. 



Hustin (1912, p. 334), in perfusion of the pancreas with saline solutions, 

 found that, if these were hypotonic with respect to the normal blood, the con- 

 centration was increased by passing through the blood vessels of the gland. If 

 hypertonic, the concentration was diminished. The explanation on the basis 

 of semi-permeability of the gland cells is simple ; these cells would take up water 

 from a hypotonic solution in order to equalise their osmotic pressure to it and 

 give up water to a hypertonic solution. No satisfactory explanation is apparent 

 on any other view. No change takes place in the composition of the perfused 

 fluid if the cells have been killed by sodium fluoride, so that their semi-permeability 

 is abolished. 



The ratio of the sugar content of blood corpuscles to that of the plasma is 

 very variable, although as a rule higher in the plasma than in the corpuscles. 

 The addition of glucose to the blood sometimes raises the content of the corpuscles, 

 sometimes not (Hober, 1912, 1). It is difficult to give an explanation of 

 these facts. It seems that the conclusion must be drawn that the corpuscles are 

 capable of being made permeable or impermeable to glucose, but that their usual 

 condition is that of impermeability. 



At this point it is well to call attention to the remarks justly made by Hober 

 (1911, p. 244) to the effect that it is impossible to account for the constant 

 difference in the ratio of potassium to sodium in the blood corpuscle and other 

 cells compared with that in the plasma, which bathes them, except on the hypo- 

 thesis of complete semi-permeability. If these salts were able to diffuse out, 

 however slowly, equilibrium must result sooner or later, unless the extremely 

 improbable assumption be made that the corpuscles and other cells obtain a 

 continuous supply of salts from some source other than the blood and that the 

 latter is able to get rid of them as fast as they pass in. 



We now come to the third set of facts proving the semi-permeability of cells 

 towards salts, namely, those connected with the electrical conductivity of cells. 

 A few preliminary words of explanation are desirable. 



When an electrical current is passed through a solution of a salt by means of wires dipped 

 into it, the transport of electricity from one wire to the other is effected by means of atoms or 

 molecules, each carrying a definite amount. These along with their charges, which ditlcr 

 according to the valence of the carrier, are called ions. The unit charge, carried by a univali-nt 

 ion, is known as an electron. A bivalent ion carries two electrons and so on. Imagine a flock 

 of sheep at one side of a field and that they start to run to the other side ; the amount of wool 

 (= electricity) which arrives at the other side in unit of time depends on the number of sheep 

 and on the freedom of the course. Suppose that there are a number of square pens in the 

 middle of the field, each fenced round and separated from the neighbouring pen by a narrow 

 interval, the number of sheep now getting across in unit time will be much less than before, 

 because they have to wait for each other to get through the openings, or rather, they obstruct 

 one another.in their efforts to get through. We may say that less wool passes across per unit 

 time, or in electrical terms, the conductivity is less. Further, matters would not be improved 

 if the closed pens were full of sheep, since these sheep would not be able to help in the 

 transport. On the other hand, suppose the cross-fences were removed, the enclosed sheep 

 could get out and cross the field, while the originally free sheep would have as clear a course as 

 if no pens were there. 



Living cells, as regards the transport of electricity, are like the enclosed 

 pens with sheep in them and are in the same way obstructive to the passage 

 of ions by filling up part of the channel. Whereas, if we make their membranes 

 permeable to salts, the resistance is removed. This fact, in the case of the 

 blood corpuscles, was described in detail by G. N. Stewart (1897) and made the 

 basis of a method of determining the relative proportion of corpuscles and plasma 

 in blood (1899). 



Osterhout (1912) also finds that living cells of Laminaria are impermeable to 

 the salts of sea water, as shown by their taking no part in the conduction of an 

 electrical current. They are made conductors by any agent which kills the 

 protoplasm, such as heat, chloroform, and so on. The'ir permeability also can be 

 changed reversibly, as will be seen later. M'Clendon (1910, p. 255) finds that the 

 eggs of sea urchins massed together have a conductivity greatly inferior to that 



