74 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



loss of function of the nerve-cells in the cord by which the muscles are nor- 

 mally excited to action. Later, however, the nerves and muscles of the limb& 

 lose their irritability. Of the peripheral mechanisms the motor nerve-end* 

 are found to succumb before the nerves and muscles. This is shown by the 

 fact that although the muscles are still capable of responding to direct irrita- 

 tion, they are not affected by stimuli applied to the nerve, although the nerve 

 at the time, to judge from electrical changes which occur when it is excited, 

 is still irritable. Since the nerve and muscle are irritable, the lack of response 

 must be attributed to the nerve-ends. The response to indirect stimulation 

 (i. e. excitation of a muscle by irritating its nerve) is lost in about twenty 

 minutes, while the irritability of the muscle, as tested by direct excitation, i 

 not lost for four or five hours. In this 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 in 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 

 (see Fatigue of Nerve, p. 79). Nevertheless, the nerve-fibre requires a con- 

 stant 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 themselves will be con- 

 sidered later. 



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

 experiments as those of Ludwig and Schmidt j 1 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 



1 Sitzungsberichte der math.-phys. Clause der k. sacks. Gesellschaft der Wissenschaften, vol. xx., 1 868. 

 1 " Versuche iiber den Stoffwechsel des Muskels," Archiv filr Anatomie und Physiologic, 

 1885; physiologische Abtheilung, p. 533. 



