From this general discussion it is quite evident that the cardiac muscles of different animals act in very different ways and that while some, like the heart of Limulus, have a neurogenic beat, others like that of the tunicates have a myogenic beat.
From this rather lengthy digression we may return to the question raised in the earlier part of this lecture, namely, the possibility of the existence of physiologically independent muscles. This I believe to have been demonstrated in part at least in the sphincter pupillæ of the lower and perhaps all vertebrates, and wholly so in the tunicate heart and the embryonic vertebrate heart. The complete freedom of such muscles from nervous control and their dependence on direct stimulation for normal action is a repetition of a process that, in my opinion, characterized all primitive muscles such as we now meet with in the sphincters of sponges. Such muscles as these sphincters I believe to represent the original and primitive elements around which the other members of the neuromuscular mechanism, the sense organs and the central nervous organs, subsequently developed. In my opinion then, effectors in the form of muscles preceded in an evolutionary sense the receptors and adjustors, and formed the centers around which these organs developed later.
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