ORIGIN OF THE NERVOUS SYSTEM 339 



ous relations, both central and peripheral, are too well known to require 

 comment. To this process must be added, I believe, the appropriation 

 of totally new muscles. There is good reason to assume that the heart- 

 beat in tunicates is of myogenic origin and the fact that the embry- 

 onic vertebrate heart pulses before it contains any nervous elements is 

 strong evidence in favor of the view that the cardiac muscle of the 

 primitive vertebrate was a muscle developed independently of nervous 

 control. That that muscle in modern adult vertebrates is under a 

 certain amount of nervous control is unquestionable, but this control is 

 not of the kind usually seen in other neuromuscular combinations. The 

 nerves that enter the heart are probably not ordinarily directly con- 

 cerned with its beat, for, as already pointed out, this continues after 

 they are cut. The function of these nerves seems to be that of modi- 

 fying this beat and in this respect two classes of fibers may be distin- 

 guished: augmentors which increase the beat, and inhibitors which 

 retard or even check it. This whole nervous mechanism has the appear- 

 ance of having been superimposed upon a muscle that was originally 

 non-nervously active, and I therefore regard the vertebrate heart as an 

 example of an originally independent muscle secondarily brought under 

 the influence of central nervous organs. Many other muscles, like the 

 sphincter pupillse, etc., have doubtless had a like history, but as they 

 have not been investigated from this standpoint, the question of their 

 exact relations to nervous control must remain for the present some- 

 what open. 



In the vertebrates at least, nervous effectors include not only mus- 

 cles, but also electric organs. These organs occur not infrequently among 

 the fishes. They are best represented in the South American electric 

 eel, the electric catfish of Africa and the torpedoes of the Mediterranean 

 Sea and the Atlantic and Indian Oceans. They also occur less fully 

 developed in certain skates, mormyres and the star-gazer. These organs 

 are usually imbedded in a mass of the fish's muscle or they occupy such 

 positions that they clearly replace muscles. Their histogenesis, as 

 worked out particularly in the skates by Ewart (1888), shows con- 

 clusively that each electric plate is a modified muscle-fiber and in fact 

 there seems to be good reason to conclude that all known electric organs, 

 excepting possibly those of the electric catfish, are modified muscles. 

 This is entirely consistent with what is known of the physiology of these 

 two kinds of effectors, for muscles not only move parts, but generate 

 through their activity a certain amount of electricity, while the electric 

 organs have lost the power of producing molar movements and have 

 enormously increased that of producing electricity. Electric organs, 

 though often described as a special class of effectors, are in reality 

 merely modified muscles and therefore can not be regarded properly as 

 a new appropriation of the nervous system. 



