578 



COMPAEATIVE ANATOMY. 



The mechanical arrangements by which the stream of blood is 

 directed are therefore, at first, essentially the same at both ostia of 



the ventricle. The cavity of the ventricle is 

 continued into the arterial trunk which is 

 given off from it, and commences by an en- 

 largement (bulbus arteriosus). That part 

 of the ventricle which is attached to this bulb 

 has an elongated form in the Selachii and 

 Chimasrae, and in its wall the musculature 

 has gained a circular arrangement. This 

 portion of the heart is the conus arte- 

 riosus (B). It is marked off from the bulb 

 by three pouch-like valves ; behind these 

 there are a number of valves in the conus 

 arteriosus, which are set in longitudinal rows. 

 There is a similar arrangement in all the 

 Ganoi'dei. In the Teleostei it is rarely pos- 

 sible to distinguish the conus arteriosus, 

 and, as a very general rule, there are only 

 two valves between it and the bulbus. In 

 Ceratodus there are rudiments of two rows of 

 valves behind the four pouched valves that are 

 placed on the boundary line of the conus. In 

 Protopterus there are longitudinal folds which 

 foreshadow a division of the cone. 



The atrophy of the conus arteriosus in the 

 Teleostei is accompanied by a development of 

 the bulb, and the smooth muscular tissue in its walls is increased in 

 quantity. It thus gets the appearance of an independent portion, 

 which must not be confounded with the conus arteriosus. In all 

 Fishes, except the Dipnoi', the heart contains venous blood only. 



Heart of 

 vulgaris. 

 The anterior wall of the 

 ventricle and of the 

 conus arteriosus is re- 

 moved. A Auricle. 

 V Ventricle. B Conus 

 arteriosus, o Ost. atrio- 

 ventricular, a Bran- 

 chial arteries. 



§ 432. 



The trunk of the branchial artery is placed in all Fishes below 

 the branchial framework, along the arches of which it sends out its 

 branches. In the earlier stages of development these pass on each 

 side directly into a longitudinal vessel which runs along the base of 

 the skull, and from which an artery is continued forwards to the 

 head, and especially to the brain and eye (carotis interna). Pos- 

 teriorly the two longitudinal trunks (roots of the aorta) are united 

 into an unpaired larger trunk — the aorta (cf. Fig. 325). When the 

 branchial lamellae are developed on the branchial arches, a vascular 

 network is gradually developed in them by the arterial arches. 

 When this network is well developed the arterial arches disappear. 

 Each of them is replaced by a network of capillaries, which is 

 supplied by a branch of the branchial artery, and which gives off 

 an artery, or branchial vein, which takes part in the formation of 



