68 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



not lost for four or five hours. In tin's as in so many instances the loss of 

 irritability of the muscle is due primarily to the disturbance of the respira- 

 tion of the muscle. Of the substances supplied to the muscle by the blood, 

 oxygen is one the want of which is soonest felt. The muscle contains within 

 itself a certain store of oxygen, but one which is by no means equal to the 

 amount of oxidizable substances. Of this oxygen, that which is in the least 

 stable combinations, and which is available for immediate needs, is soon 

 exhausted. A continual supply of oxygen is required even for the chem- 

 ical changes which occur iu the quiet muscle. Of the waste substances which 

 the blood removes from the cell, carbon dioxide is the one which accumu- 

 lates most rapidly and is the first to lessen the irritability. Lactic acid and 

 waste products from the breaking down of nitrogenous materials of the cell 

 are also injurious. 



The dependence of nerve-fibres upon the blood-supply is by no means so 

 well understood. The nerve-fibre is a branch of a nerve-cell, and it seems as 

 if the nourishment of the fibre was largely dependent upon that of the cell- 

 bodv (see Fatigue of* Nerve, pp. 75 and 95). Nevertheless, the nerve-fibre 

 requires a constant supply of blood -for the maintenance of its irritability. 

 The irritability of the nerve cannot long continue without oxygen, and a 

 nerve which has been removed from the body is found to remain irritable 

 longer in oxygen than in air, and in air than in an atmosphere containing no 

 oxygen. Waste products liberated by active muscles have a deleterious 

 effect on nerves ; whether such substances are produced in the nerves them- 

 selves will be considered later. 



The efficacy of the blood to preserve the irritability is to be seen in such 

 experiments as those of Ludwig and Schmidt ;* they succeeded in maintaining 

 the artificial circulation of defibrinated, aerated blood through the muscles 

 of a dog, and kept them irritable for many hours after death of the animal. 

 If such an experiment is to be successful, the blood must be maintained at the 

 normal temperature, be plentifully supplied with oxygen, and be kept as free 

 from carbon dioxide as possible. Von Frey 2 made an elaborate experiment of 

 this nature. A dog was killed, the body was cut in halves, and the aorta and 

 inferior vena cava were quickly connected with an apparatus for pumping the 

 blood at a regular rate through the hind part of the body. Before the blood 

 entered the arteries it passed through coils in which it was warmed to the nor- 

 mal temperature, and an artificial lung, where it received a supply of oxygen 

 and was relieved of its carbon dioxide. Under these conditions the muscles 

 were kepi alive for more than seven hours, and so far retained their normal 

 condition that throughout this period they were able to respond to stimuli 

 sent to them through their nerves and contract with sufficient vigor to raise a 

 considerable weight. H. X. Martin 3 made a similar experiment on the heart 



1 SUzungaberichte der math.-phys. Claisse der k. sacks. Gesellschaft der Wissenschaftm, vol. xx., 1868. 



2 " Versuche iiber den StoflTweohsel des Muskels," Archiv fur Anatomic und Physiologic, 

 1885; physiologische AbtheilunK, S. 533. 



3 Studies from the Biological Laboratory of Johns Hopkins University, 1882, vol. ii. p. 188. 



