264 



LECTURE X. 



Commencement of Systemic Circulation ; 

 Gadus Ciillarms, Miiller. 



outwards to the end of the uniting substance, and there subdivide, 

 (in the Cod,) one twig extending along the internal margin of the 

 branchial process to its extremity, the other retrograding along 

 the same margin to its base : from these marginal twigs the minute 

 transverse vessels are distributed to the fine branchial laminae upon the 

 sides of the process. The arterialised blood is carried to an efferent 



vessel, returning along the ex- 

 ternal margin of each branchial 

 process, and by these is poured 

 into the branchial vein {fig. 69- 

 b). The four veins on each side, 

 which are analogous to the pul- 

 monary veins in man, vniite to 

 form the ' aortic circle' (ib. a) 

 which encompasses the basi- 

 sphenoid (h). The current of 

 arterialised blood flows forward 

 at the fore-part of this circle 

 into the hyo-opercular (e') and orbito-nasal {b) arteries ; but the main 

 streams are directed backwards, and converge in the direction of the 

 arrows to the aortic trunk. The carotids (c), the homologues of the 

 subclavians (c?) sent to the pectoral fins, and sometimes the coronary 

 vessels of the heart, are sent off from the aortic circle. But no 

 systemic heart or rudiment of a propelling receptacle is developed in 

 any fish at the point of confluence of the branchial veins. 



Small vessels are sent off from the marginal branchial venules 

 by short trunks, which ramify beneath the branchial membrane, and 

 become the ' arteria3 nutritire' of the gills : their capillaries are col- 

 lected into venous trunks, which quit the gills commonly at both 

 their extremities, those from the dorsal ends joining the jugular 

 veins, those from the ventral ends emptying themselves into the prie- 

 cavals, or directly into the great auricular sinus. * 



Such is the outline of the general structure of the beautiful and 

 complex mechanism of the normal or pectinated gills of fishes. 



There are many minor modifications of this form, some of which I 

 am tempted to notice from the explanation of their ' physical cause,' 

 which they receive from the known phenomena in the development 

 of the gills ; or from the light which the known habits of the 

 species throw upon their ' final purpose.' But, first, a brief sketch 



* These ' vena nutritias ' are unusually large in the Carp ; but are not, as Dii 

 Verney supposed (cviii.), directly continued from the true ' vente braiichiales ; ' 

 and they do not, therefore, divert any of the stream of arterialised blood from the 

 aorta to uoiir it directly into the venous sinus. See Miiller, xxi. 1841, p. 'IS. 



