INDEPENDENT EFFECTORS 53 



ably also that of the cephalopod eye, may, therefore, be 

 regarded as muscles normally subject to direct stimula- 

 tion by light, notwithstanding the fact that they are also 

 under nervous control. In so far as this type of muscle 

 is open to direct stimulation, it reproduces the condition 

 found in sponges where a superficial sphincter is also 

 the form assumed by the muscle in question. Whether in 

 addition to stimulation by light the sphincter pupillaB in 

 vertebrates is open to direct chemical stimulation, as 

 might be inferred from the work of Auer and Meltzer 

 (1909) on calcium, remains still to be definitely decided. 



Although the sphincter in the iris of the eye appears, 

 therefore, to be a muscle normally subject to direct stim- 

 ulation, and in this sense illustrates the principle of an 

 independent effector, the muscle that has been most dis- 

 cussed from this standpoint is that of the vertebrate heart. 

 In general two opinions have been held respecting the 

 cause of contraction in this muscle. According to one 

 view, the myogenic theory, the beat of the heart is brought 

 about through changes that are initiated in the heart 

 muscle itself. According to the other view, the neuro- 

 genic theory, the impulses to rhythmic beating originate 

 in nervous tissue and are delivered secondarily to the 

 heart. The myogenic theory implies that this muscle is 

 essentially an independent effector not unlike the muscu- 

 lar organs of sponges. The neurogenic theory places the 

 heart muscle under nervous control, as most of the 

 muscles in the higher animals are. 



These two theories of the nature of the heart-beat have 

 had a long and complicated history, which has been well 

 reviewed by Engelmann (1904) and by Howell (1906). 

 It will, therefore, not be necessary in the present connec- 

 tion to enter into this side of the subject beyond stating 



