io8 THE INVOLUNTARY NERVOUS SYSTEM 



heart resembles closely the musculature of the Limulus heart. 

 In both cases the musculature possesses rhythmic power, but 

 the normal beat does not occur when the muscle is first sepa- 

 rated from the motor nerve cells. When started it may, how- 

 ever, persist for a long time, for Stefanowska found it beating 

 six to eight days after destruction of the spinal cord. Finally 

 Tschermak has shown that these cells in the case of the lymph 

 heart maintain a tonic not a rhythmic influence on its musculature, 

 for there is no sign of any periodic movements in a delicate 

 capillary electrometer when it is connected with the central end 

 of the nervus coccygeus superior. 



So far the evidence appears to me to show that the nerve 

 cells within an organ such as the heart (the intrinsic cells) cannot 

 be looked upon as different in character from those not imbedded 

 in the organ (the extrinsic cells). 



Following the heart come the blood vessels, in which well- 

 defined nerve cells are few and far between : that such do occur 

 is clear from Jegorow's picture, fig. 8. He describes also by 

 the methylene blue method a network of nerve fibres closely 

 surrounding the muscular coat of the blood vessels, but does not 

 figure nerve cells in this network. Langley has shown that 

 this network entirely degenerates when the motor nerves to 

 muscles of the blood vessels have been cut. In the meshes of 

 the network nodal thickenings have been described and have been 

 called nerve cells ; but they are unable to keep alive the network 

 after the separation of the extrinsic motor nerves. 



The same also holds in the allied case of. the dermal system 

 of involuntary muscles. We may take as an example of these 

 muscles the Retractor penis, as that muscle has been more care- 

 fully examined than any other. Its motor fibres arise from cells 

 in ganglia of the lumbar chain, and its inhibitory fibres from cells 

 in the pelvic ganglia : both these sets of ganglia are extrinsic, 

 lying outside the muscle. In the muscle itself Fletcher finds a 

 well-marked nervous network surrounding the muscle fibres. In 

 this network there are no nerve cells, and when both the nerves 

 are cut this network disappears : there are no intrinsic nerve 

 cells to keep it alive. In all probability the same is true of the 

 whole of the dermal musculature. 



The most striking case of a regular normal rhythm in an 

 artery is to be seen in the arteries of the leech. My son, J. F. 



