VASCULAR SYSTEM OF FISHES. ' 2o5 



semilunar valves at the ' ostium arteriosum ' behind its origin, beyond 

 which it slightly dilates, but has no muscular parietes constituting a 

 ' bulbus arteriosus.' In this IJunterian preparation of a large Myxinoid 

 fish (No. 1018.) the artery divides at once into two branchial trunks, 

 reminding one of the separate branchial arteries of the Cephalopods. 

 In other species of Bdellostoma the artery extends beyond two or 

 three pairs of gills before it bifurcates; and Miiller* saw one in- 

 stance in the Myxine glutinosa, where the branchial artery continued 

 single as far as the anterior gills. 



The pericai'dium of the Anmiocete communicates by one wide orifice 

 with the peritoneum : that of theLamprey is a shut sac, and is supported 

 by a perforated case of cartilage, formed by the last modified pair of 

 branchial arches {fig. 11. 47', p. 52.). Not any of the Dermopteri pos- 

 sess the ' bulbus arteriosus :' this is present, and forms, as it were, a 

 third compartment of the heart, beyond the ventricle and auricle in all 

 other Fishes {fig' 61. r. fig. 70. c) : nay, if we include the great ' sinus 

 communis' as part of the heart, then we may reckon four chambers 

 in that of Fishes ; but the student will observe that these succeed each 

 other in a linear series, like the centres of the brain, and their valves 

 are so disposed as to impi-ess one course upon the same current of 

 blood from behind forwards, driving it exclusively into the branchial 

 artery and its ramifications. This is very different from the arrange- 

 ment and relations of the four compartments of the human heart. 

 Physiologically the heart of Fishes answers to the venous or pul- 

 monary division, viz. the right auricle and ventricle of the mammalian 

 heart, and its quadripartite structure in Fishes illustrates the law of 

 vegetative repetition, rather than that of true physiological complica- 

 tion.f The auricle and the ventricle are, however, alone proper to 

 the heart itself: the sinus is a development of the termination of the 

 venous system, as the muscular bulb is a superaddition to the com- 

 mencement of the arterial trunk. 



Some of the higher organised Fishes, which present the normal 

 structure of the heart, have, like the Myxinoids, a perforated peri- 

 cardium. In the Sturgeon the communication with the peritoneum 

 is by a single elongated canal extending along the ventral surface of 

 the oesophagus. In the Planirostra and Chima3roids the pericardio- 

 peritoneal canal is also single. Li the Plagiostomes it bifurcates, 

 after leaving the pericardium, into two canals, which diverge and 

 open into the peritoneum, opposite the end of the oesophagus : no 

 ciliary movements have been noticed on the surface of these remark- 



* XXI. ib. p. 9. 



f The heart of Fishes with tlie muscular branchial artery will he seen to be the 

 true 'homologue' of the left auricle, ventricle, and aorta in higher Vertebrata, as wo 

 trace the complication of the heart synthetically; but it performs a function 'ana- 

 logous' to that of the pulmonic auricle and ventricle in them. 



