CHAP. IX.] THE CONTRACTILE TISSUES. 385 



disturbances of tetanus ; but in what direction they are influenced, 

 whether they are checked or accelerated, is not clear. During 

 tetanus the blood becomes deprived of oxygen and charged with 

 carbon dioxide. Both these circumstances are unfavourable to the 

 gaseous exchanges of tissues generally ; nor are they compensated 

 by an increased respiratory and circulatory activity, for on prolonged 

 tetanus general asphyxia may arise 1 . 



These considerations do not, however, entirely make clear the 

 origin of the carbon dioxide; and the uncertainty must always be 

 kept in mind when conclusions obtained in a study of the general 

 exchanges of the body are applied to muscles alone. 



The absorp- A small correction is necessary in respect to the oxygen 

 tion of o dis- absorbed. After tetanus, the blood generally contains 

 cussed. a &m aller proportionate quantity of oxygen. This defect 



is to be ascribed to tetanus just as much as the defect of oxygen from 

 the air inhaled ; and hence it must be added to the latter before 

 the oxygen absorbed during tetanus can be compared with the carbon 



CO 



dioxide excreted. The effect of this addition is to make the quotient -~ -* 



somewhat smaller than before ; but in no remarkable degree. For, 

 assuming the blood to contain 13 per cent, of oxygen, and half of 

 it to be found wanting at the close of a short experiment, it would 

 merely raise the oxygen absorbed by about 1 2 c.<j. per minute of 

 the experiment, in the case of a rabbit of 2 kilogs. 



Effects of While the changes of respired air which are 



muscular brought about by muscular exertion are so pronounced 



exercise on a s to have been remarked by the earliest observers, and 

 secretion ^ moreover give a decisive indication of the changes 

 which muscular exercise works in the blood, the changes 

 of the urinary excretion under the same circumstances have been 

 a matter of continual uncertainty. An examination of the methods 

 of the earlier investigators discloses the causes of this uncertainty. 

 The urine the great drain of effete nitrogenous subtances although 

 it is beyond doubt affected by the activity of the eminently nitro- 

 genous substance of muscle, is affected to an unexpectedly small 

 degree. In a word, the urine, in so far as it represents the tissue- 

 waste of muscle, is dependent upon the nutritional rather than 

 upon the functional processes of muscular tissue. Moreover nitro- 

 genous muscle, although an important source of the nitrogenous 

 excreta of urine, is by no means the only source. The character 

 of the food, its composition, the proportion of its elements, the 

 time of its ingestion, are all conditions which exert a large influence 

 over the constitution of the urine. It is to these two circumstances, 

 viz. to the smallness of the change which muscular exercise effects in 

 the urine, and above all to the omission from the calculation of the 



1 Sczelkow, Op. cit. 

 G. 2$ 



