538 EEPOET OF COMMISSIONER OF FISH AND FISHERIES. [84] 



the living heart nor yet in sections of ifc have I succeeded in making it 

 out; at most it must be very thin, like the pericardiac membrane itself. 

 Moreover, the whole of the cardiac walls are contractile, as seen in the 

 living embryo. The contractility of the heart at an early stage is a very 

 remarkable phenomenon, in that it as yet contains no clearly marked, 

 spindle-shaped muscular cells or fibers; the contractility manifested 

 during its early phases is apparently almost an automatic process, the 

 stimulus for which may or may not proceed from any splanchnic nerves. 

 It would appear that the nervous system during this early stage was 

 hardly well enough developed to take part in the stimulation of the 

 organ. I never saw any knot of nervous matter on it, or in its vicinity, 

 which I can identify as a cardiac plexus or ganglion, and traceable as 

 an appendage of the tenth or vagus nerve. 



With the advance of development the blood is also formed, but not 

 until about ten days after hatching; meanwhile, the heart has also as 

 snmed its final position in the median line; the venous end is no longer 

 swung slightly to the right side, but occupies an antero-posteriorly 

 inclined position in the middle line, as shown in Fig. 49. Its walls, 

 especially those of the ventricle h, have become thicker, but there are 

 as yet no muscular pillars or partial septa developed in it, as we find in 

 the ventricle of Cottus and AJosa. The valves, also, cannot be said to 

 be developed as folds, such as we see in the adult. The office of the 

 valves seems to be performed solely by the rhythmical constriction of 

 the cardiac tube at definite points. First, the venous sinus fills with 

 blood, which is still pale and comparatively rich in serum; then the 

 atrio-ventricular valvular constriction opens, and the blood is forced, 

 by the contraction of the walls of the sinus, into the ventricle, when 

 the intervening constriction again closes, and confines the blood in the 

 ventricle, from whence it passes, by a similar process, into the bulbus, 

 and so into the branchial vessels. I do not mean to imply, however, 

 that the contracted portions of the heart-tube do not mark the regions 

 where valves will appear in future, nor to convey the impression that 

 the regions where the rhythmical valvular collapse of the cardiac tube 

 occurs during pulsation are not constant in the embryonic hearts of 

 young fishes. All of the cardiac compartments appear to exhibit pul- 

 sations even to the bulbus, which, according to Huxley (Anat. Verte- 

 brates, 140), is not rhythmically contractile in the adult. True, the 

 walls of the embryonic bulbus are thin, like those of the sinus, and it 

 may be that its dilatations and contractions are simi^ly an efi'ect of the 

 distension produced by the rhythmical contractions of the ventricle. 

 The effect of the pulsations of the ventricle upon the blood current are 

 visible at a late stage, just as we see in the circulation of the gills of the 

 salamander or the web of a frog's foot; what is here meant is tliat the 

 blood flow is not at a uniform rate, but the current in the vessels moves 

 slightly slower and faster alternately, owing to the alternate exertion 

 and non-exertion of the propelling power of the heart during the diastole 



