4 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the plexus, but he found that each of the great nerve- 

 cords descending from the plexus into the limb is itself a 

 plexus, that defies dissection. 



Histology (microscopic anatomy) and physiology can, 

 however, by reason of their finer methods overcome 

 the difficulty thus offered. The physiologist, John Miiller, 

 discovered early in this century the "law of isolated 

 conduction" of nerve-fibres. If, therefore, a spinal nerve 

 be excited, e.g., by an electric current conveyed to it as it 

 leaves the spinal cord before it enters the plexus, the fibres 

 that are contributed by it to the various nerves of the limb, 

 will be excited and no other fibres, however close its own 

 may run to them. 



The functions of the arm and hand are, however, so 

 multitudinously various that it is well to choose some 

 single, simple one, and to regard it only, letting it stand 

 as a concrete instance and subject for the physiological 

 part of the enquiry. Such an instance may well be chosen 

 in the simple movement I perform when holding my arm 

 out horizontally I flex my fingers, folding them into the 

 palm. The question before us therefore is how is that 

 movement performed, and how does the morphological 

 structure of the part assist us toward explaining in what 

 ;way the mechanism is actuated ? 



The flexion of the fingers is due to the contraction of 

 certain muscles situated on the ventral aspect of the forearm 

 and in the palm. In order to simplify our problem still 

 further, let us exclude the palmar muscles. The move- 

 ment can certainly be performed — as has been proved 

 experimentally in the monkey — without the contraction of 

 the palmar muscles, and this is a further justification for 

 simplifying our problem by their exclusion. The 

 muscles in question, in the forearm, are thrown into con- 

 traction by the action of the nerves which pass to them 



