778 DR PETTIGEEW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



malia, it will be evident that there is nothing, either in their structure or rela- 

 tions, to betoken any great degree of activity on their part. That these structures 

 are, on the contrary, principally passive, seems certain from the fact, that a 

 stream of water or other fluid directed upon them from above as recommended, 

 at once closes the orifices which they guard. 



Stkuctuke of the Bulbus arteeiosus of the Fish, and of the Ventricle of the Fish 

 AND Eeptile ; Semiltjnar and other Valves found therein. 



The semilunar valves in the bulbus arteriosus of the fish, and the auriculo- ven- 

 tricular valves in the fish and reptile, differ from the venous and arterial ones, 

 in being, for the most part, connected either directly or indirectly, or exposed in 

 some way to the influence of muscular contractions. In order the better to 

 understand the position which these valves occupy in the gradually ascending 

 scale of valvular arrangements, a brief description of the bulbus arteriosus, of 

 the fish^ and of the ventricle of the fish and reptile, is necessary. In the ventricle 

 of the fish, the fibres, as I have pointed out elsewhere,* consist of three layers ; — 

 an external layer, in which they proceed from base to apex, and occasionally 

 interdigitate and become strongly embraced ; an internal layer, in which they 

 are aggregated into fascicular bundles, and have a more or less vertical reticu- 

 lated arrangement ; and a central layer, in which they run transversely, or at 

 right angles to the fibres of the external and internal layers. These layers are 

 connected to each other by certain fibrous bands, which run in a direction from 

 without inwards. Rising from the base of the ventricle anteriorly is a muscu- 

 lar structure of a more or less bulbous form, the so-called bulbus arteriosus 

 (Plate XXIX. figs. 38, 39, 40, 41, and 48), the arrangement of the fibres in 

 which, resembles that in the ventricle itself. The ventricle of the fish, and 

 the bulbus arteriosus contract in every direction, and in this respect they are 

 analogous to the veins and arteries, which, as John Hunter showed, are exten- 

 sible and retractile, both in their length and breadth. One point to be noted in 

 the ventricle of the fish, is the absence of musculi papillares ; the auriculo-ventri- 

 cular valves being so placed, that certain of the fasciculi constituting the internal 

 layer, run parallel to them, and extend, in not a few instances, into their sub- 

 stance. The effect of this arrangement is to modify the action of the valves in 

 question ; and I direct attention to the circumstance, because of the purely 

 mechanical views entertained by some with regard to them ; views which to me 

 appear inconsistent with the nature of the textures involved. The arrangement 

 of the fibres in the ventricle of the reptile is nearly the same as that in the 

 fish. There is, however, this difference, and it is worthy of mention as bearing 

 directly upon the structure and function of the auriculo-ventricular valves in this 



* On the Arrangement of the Muscular Fibres, in the Ventricles of the Vertebrate Heart, with 

 Physiological Remarks. — Phil. Trans., vol. 154, pp. 445-47. 



