170 GENERAL PROPERTIES OF" LIVING TISSUES 



Muscle with Nerves Degenerated. A nerve 

 fibre severed from its cell of origin dies or " de- 

 generates " down to its ultimate endings. Expose 

 the sciatic nerve in the middle of the thigh of a 

 frog in which the nerve has been severed near 

 the pelvis ten days before, so that the whole of 

 the nerve distal to the section shall have degen- 

 erated. Stimulate the degenerated trunk. 



No contraction is seen in the muscles of the 

 leg. Stimulate the muscles directly. 



Contraction takes place. 



The Nerve-free Embryo Heart. Embryological 

 studies show that the nerves of the heart are 

 formed from epiblast in the walls of the neural 

 canal, and do not grow into the heart until the 

 close of the third day of incubation (chick). 

 The heart, however, begins to beat during the 

 second day of embryonic life, before even the 

 blood which it shall pump is formed. Thus 

 the heart muscle, in the embryo, is capable of 

 contraction in the absence of nerves. 



Cover an egg which has been incubated 60-70 

 hours with 0.75 per cent solution of sodium 

 chloride warmed to 38 C. Kemove the shell 

 with the forceps over one third of the egg, be- 

 ginning at the broad end, and leaving the shell 

 membrane behind. Now remove the shell mem- 

 brane. Note the beating heart. 



