306 PHYSIOLOGY CHAP. 



fibres from more than one root, i.e. they are polymeric, belonging 

 to many myotomes. On the other hand, each myotome contains 

 portions of several muscles. The actual muscles, derived from the 

 fusion of several monomeric units, may be classed in three groups : 



(a) Muscles that remain monomeric, with a single function. 

 Among these are the small vertebral muscles above referred to. 



(b) Polymeric muscles with a simple function, as the rectus 

 abdominus, which is innervated by the 5th-12fch thoracic roots ; 

 the tendinous bands seem an evidence of the fusion of the eight 

 segments of which the muscle is composed. 



(c) Polymeric muscles with complex functions. Most of the 

 skeletal muscles belong to this category. 



When a muscle thus receives fibres from two ventral roots, 

 does the stimulation of one of these roots produce total contraction 

 of the muscle ? Sherrington replies in the affirmative ; he even 

 maintains that it is not necessary to stimulate the whole of the 

 root ; it suffices to excite any one of the filaments or rootlets 

 which compose the root, as it passes through the dural sac, in 

 order to throw the entire muscle into contraction. Kisien Eussell 

 contradicts this emphatically, and affirms that stimulation of a 

 single root of a polymeric muscle only throws a portion of it into 

 contraction. This is obviously the case for the sartorius muscle. 

 Whatever the final solution of this controversy, it is certain that 

 although a myotome may be a complex of muscle fibres which 

 have only a single function, it is far more frequently found that 

 the muscular complex of the myotome contains elements with 

 antagonistic functions. In this case it is evident that the same 

 ventral root must contain separate fibres for both functions. 

 Thus Martin and Hartwell observed in the dog a rhythmically 

 alternating functional activity of the motor root which innervated 

 the antagonistic internal and external intercostal muscles. 



The physiological unit of cutaneous metamerism the derma- 

 tome has recently been the subject of a careful experimental 

 study by Winkler and Van Rynberk. They found that the 

 dermatome consists of two areas, one central, the other marginal. 

 The former is capable of maintaining sensibility even when all 

 overlapping is abolished by section of the neighbouring posterior 

 roots ; the latter, on the contrary, is not capable of subserving 

 sensibility without the co-operation of the overlapping dermatomes 

 (Fig. 182, A). 



The sensibility of the central and marginal areas of the 

 dermatome is not uniform, but varies in degree at different points. 

 Three spots can be distinguished in the dermatome at which 

 innervation and therefore sensibility are maximal. One of these 

 lies near the dorsal median line, the second near the lateral line, 

 the third near the ventral median line, as shown in the diagram. 

 From these points, at which it is most acute, sensibility diminishes 



