vi MOEPHOLOGY OF THE HEAKT 373 



certain levels the increase in diameter is much less pronounced than 

 it is elsewhere, with the result that the tube appears to be 

 constricted at these points while it bulges out between them. This 

 development of a series of dilated portions of the heart-tube is the 

 first step in its segmentation into a series of chambers. Of these 

 chambers there are typically in the lower vertebrates four — sinus 

 venosus, atrium, ventricle and conus arteriosus. 



Allusion must be made in passing to an unfortunate confusion 

 of nomenclature which is apt to prove a stumbling-block in the 

 way of the student who is trying to get his ideas clear regarding 

 the morphology of the heart. The name conus arteriosus was first 

 used, so far as the comparative anatomy of the lower Vertebrates 

 is concerned, by Gegenbaur (1866) who used it to designate the 

 structure lying between ventricle and ventral aorta in Elasmobranchs 

 and Ganoids, and characterized by its possessing a muscular, 

 rhythmically contractile wall and by its containing longitudinal 

 rows of pocket valves. The name was introduced in order to 

 accentuate the supposedly fundamental difference, already suggested 

 by Johannes Miiller (1845), between the structure in question and 

 the bulbus arteriosus of Teleostean fishes. This latter is not 

 provided with striped muscle in its wall, it is not rhythmically 

 contractile : in other words it does not form physiologically a part 

 of the heart. Objections, and quite valid ones, have been raised 

 against the use of Gegenbaur's name from the side of Human 

 anatomy, it being pointed out that the " conus " of the lower fishes 

 corresponds rather with the " bulbus " of the human heart. Human 

 anatomists working at the embryology of the vertebrate heart in 

 consequence commonly use the name bulbus cordis for the part of 

 the heart under discussion. Gegenbaur's name however has come 

 to be so universally used by comparative anatomists in reference to 

 the heart of the lower vertebrates as to indicate the desirability of 

 using it in a work on comparative morphology such as this. It will 

 be understood then that the name conus arteriosus is used in this 

 volume as equivalent to what is by many writers termed bulbus 

 cordis, 1 without prejudice however to the question whether or not 

 Gegenbaur was justified in his belief that conus arteriosus and 

 bulbus arteriosus are fundamentally distinct structures. 



Elasmobranchii. — The Elasmobranch heart passes through the 

 typical early stages, first as a straight tube (see Chap. XL), then as 

 a tube which bulges towards the right side, and then as a tube with 

 the characteristic S-shaped double flexure already alluded to (Fig. 

 178). The three limbs of the S during further development become 

 converted into (1) atrium with sinus venosus, (2) ventricle, and (3) 

 conus arteriosus. The well-marked constriction which demarcates 

 atrium from ventricle forms the auricular canal. The progress 



1 I avoid in this book using the term truneus arteriosus as it is unnecessary and 

 is liable to cause confusion owing to the want of precision with which it is 

 commonly used. 



