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 Eemak 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- 



