1885.] of the Heart in Fishes, especially that of the Eel. Ill 



forming the floor of the auricular chamber is very apparent; and 

 when the fibrous threads attaching the external surface of the auricle 

 to that of the ventricle around the mitral orifice are divided, the blue 

 intervening tubular vessel is plainly seen. The connexion of this 

 vessel (canalis auricularis) with the ventricl J is seen, under the micro- 

 scope, to be chiefly established by connective tissue. There is, however, 

 muscular continuity as well ; for the ventricular substance and the 

 muscular wall of the above-mentioned vessel (canalis auricularis) are 

 connected by an extremely narrow and prolonged isthmus of muscular 

 tissue. 



II. On the Spontaneous Rhythm of the Heart as a whole and of its 

 Various Parts. 



The normal contraction of the heart begins by a distinct simulta- 

 neous beat in the right and left jugular veins, near their termination 

 in the sinus. This spot may be denominated the " ostial part " of the 

 sinus. Thence the contraction passes along the remaining part of the 

 sinus to the auricle, and then to the ventricle. The mitral orifice 

 being placed, not at the upper end of the ventricle, but a little above 

 the middle of its dorsal aspect, the contraction of the ventricle begins 

 at this part, and affects the middle portion of the ventricle before it 

 passes over the upper and lower ends. 



The normal order of contraction seen in the heart can easily be 

 reversed by direct stimulation of the ventricle. Contraction then 

 begins at the stimulated point and spreads over the whole heart ; the 

 ventricle first contracts, then the auricle, and lastly the sinus. This 

 reversed mode of contraction usually ceases to be seen whenever the 

 stimulation of the ventricle is discontinued ; in some instances, how- 

 ever, it persists for a short time longer. 



The passage of a constant current through the ventricle is some- 

 times able to cause that part to contract first, and so to lead to a 

 reversal of the normal order of contraction of the parts of the heart. 



During the continuance of the reversed mode of cardiac contraction, 

 the circulation becomes much obstructed ; the auricle and sinus 

 are distended with blood. For the auricular beat occurs before the 

 ventricular beat has ended, and the auricle is unable by its systole to 

 force the blood into a contracted ventricle. 



A regular contraction of the auricle leads very readily to a beat of 

 the ventricle. A partial contraction affecting only the part of the 

 auricle remote from the ventricle is not followed by a ventricular con- 

 traction; a partial contraction affecting the part of the auricle 

 adjacent to the ventricle is usually followed by a ventricular beat, 

 just as if a regular beat of the whole auricle had occurred. 



But apart from the occurrence of a preceding auricular beat, a 



