ORIGIN OF THE NERVOUS SYSTEM 61 the active phase of the heart-beat was during contraction, not during expansion, as had been generally supposed, and that the heart was in reality a muscular force pump. Harvey seems likewise to have had the idea, though perhaps not very clearly expressed, that the heart-beat was dependent upon the heart-muscle and not upon some extra-cardiac mechanism. In this sense he may be regarded as the founder of the myogenic theory. Later Willis pointed out that the stomach, intestine, and heart received nerves from the brain and he believed that the movements of these parts were controlled by such nerves ; he therefore may be looked upon as the originator of the neurogenic theory. To account for the fact that the heart would continue to beat for some time after its removal from the body, it was assumed by the neurogenists that the branches of the nerves left in the substance of the heart when this organ was cut from the body were sufficient to maintain the heart- beat for some time, but Haller opposed this view and declared that the heart-muscle itself was directly stimulated by the blood that coursed through it. The older form of the neurogenic theory, however, was entirely swept away by the discovery of the brothers Weber that the vagus nerve when stimulated, instead of increasing the heart-beat brought this organ to a standstill. At about this time Remak de- scribed nerve ganglia within the substance of the heart and these have been accepted by the modern neurogenists as the nervous mechanism for the heart-beat. The fact that it is practically impossible to get adult, vertebrate heart-muscle free from nerve-cells has left the prob- lem of the heart-beat in these animals in a situation difficult for ex- perimental approach. That the heart-muscle in vertebrates is always a continuous one, the auricles and ventricles being connected by at least a slender bridge of muscle, favors the myogenic theory, as does also the fact that the beat can be reversed in that the ventricle can be made to contract first and the auricle afterwards. In fact the general proposition, clearly expounded by Gaskell (1900), that the vertebrate heart is a muscular tube over which a myogenic wave of contraction proceeds from the posterior to the anterior end, has much in its favor and yet there are facts enough to show that the neurogenic interpre- tation of the action of the adult vertebrate heart is not an impossibility. The unfavorable conditions that surround the study of the verte- brate heart have forced investigators to seek evidence concerning the nature of the heart-beat in other animals and as a result two remark- ably clear sets of cases have been obtained. The first of these is the heart of the king-crab, Limulus. The heart of this animal, as Carlson (1904) has pointed out, possesses the unique feature of a complete ana- tomical separation of nervous and muscular parts. The heart itself is a long, segmented, muscular tube situated near the dorsal line of the animal. On the dorsal face of the heart is a median nerve-cord contain-
Page:Popular Science Monthly Volume 75.djvu/65
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