10 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



I have given recently a synopsis of the spinal nerve- 

 supply of the muscles of the upper limb, which shows 

 that the muscular tissue of the limb is arranged in a 

 number of rays, a ray for each metamer contributing to 

 the limb. It shows that of these rays the ones which 

 lie tailmost in the fore-and-aft series are the longest, that 

 is, extend to the extreme free apex of the limb, whereas 

 the foremost, the most rostral, pass only as far as the 

 upper arm, the next only as far as the elbow, the next 

 only as far as the wrist. The four last, or most aboral rays, 

 all contribute to the musculature of the hand in Macacus 

 rJiems, the common Rhesus monkey. 



But when we come to enquire how these units of the 

 segmental architecture of the limbs, these muscular rays, 

 are related to the physiological or functional units of the 

 limb-musculature, it is at once obvious that the extent 

 and boundaries of the two do not coincide. The individual 

 muscles of the limb are functional elements of its structure 

 as a physiological machine. But each of these functional 

 elements is compounded of portions of several rays or 

 myotoms. Moreover, the boundaries between the 

 myotoms do not correspond with the intervals of muscles, 

 or even between muscle-groups. 



The same want of segmental separation is evident when 

 we examine the location of the nerve-cells which innervate 

 the individual muscles, Each of the segmental spinal 

 nerves which supplies the musculature of the limb leaves 

 the spinal cord in the form of a row of rootlets, which 

 together build up the motor-root of the nerve. Excitation 

 by electric currents of anyone of these rootlets or fila- 

 ments, that contribute to the formation of the motor-root, 

 produces a movement in the limb which resembles closely 

 the movement produced by excitation of the whole root. 

 The same muscles are thrown into contraction as when 



