A factor of particular interest is the supply of oxygen, for we know its fundamental importance in all aërobic organisms in the breaking down of the living substance. The life of these organisms is primarily dependent upon the supply of oxygen from without. Organic reserve substances for restitution after disintegration are contained in ample quantity in the reserve stores in the living cell substance, whereas oxygen is present in very small quantities in relation to the former. It is, therefore, self-evident that the rapidity of the breaking down processes is very closely dependent upon the amount of available oxygen at hand. Nevertheless it is not the absolute quantity but the relative amount of oxygen in relation to the momentary requirement which is of importance. For instance, the quantity of oxygen present may completely suffice for the oxydative disintegration in the metabolism of rest or at lower temperature, whereas the same amount would be much too small to meet the demand increased by excitation or at higher temperature. In the latter case “_a relative deficiency of oxygen_” occurs. I have introduced the term “_relative deficiency of oxygen_”[139] for I have found that a number of authors by neglecting the relations of the available oxygen to that which is required at the moment have been led to false conclusions. There is no living object so preëminently fitted to demonstrate in such a striking manner the dependence of the duration of the refractory period upon the supply of oxygen as the spinal cord centers of the frog, when their irritability has been increased to the maximum by strychnine.[140] Various observers, such as _Loven_, _Buchanan_, _H. von Baeyer_ and others, investigated the action current by the capillary electrometer. As a means of studying the number of impulses in the strychnine tetanus, we can upon the basis of their figures roughly assume the number of impulses to equal ten per second at room temperature. In short, in the freshly strychninized frog the duration of the refractory period is about .1 second. By means of the method of artificial circulation already mentioned a deficiency of oxygen can readily be brought about. It has been demonstrated that the rhythmic in contrast to the continuous method of introduction of circulatory fluid is superior in that the former reproduces more closely the natural conditions of the circulation of the blood and renders the smallest capillaries more permeable. In consequence I have recently constructed a small appliance for artificial circulation, which accomplishes this in a manner as simple as it is complete. (Figure 31.) [Illustration: Fig. 31.