Page:Popular Science Monthly Volume 75.djvu/343

Rh 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 heartbeat in tunicates is of myogenic origin and the fact that the embryonic 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 concerned with its beat, for, as already pointed out, this continues after they are cut. The function of these nerves seems to be that of modifying this beat and in this respect two classes of fibers may be. distinguished: augmentors which increase the beat, and inhibitors which retard or even check it. This whole nervous mechanism has the appearance 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 pupillæ, 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 somewhat open.

In the vertebrates at least, nervous effectors include not only muscles, 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 conclusively 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.