STRUCTURE OF PULMONARY ORGANS OF FISHES. 81 



arranged smooth muscular fibres, which, near the point of 

 constriction of the sac, expand into a complete annulus ; whilst 

 in the posterior segment of the sac two longitudinal bands of 

 smooth muscle with transverse fibres are present, lying in the 

 most external part of the most external layer. Other fishes 

 again, as, for example, Esox lucius, Gadus callarias, and Perca 

 fluviatilis, exhibit a continuous thin layer of smooth muscular 

 fibres in the internal layer.* Lastly, the musculature may 

 be altogether absent, as in Cobitis and others. 



In some fishes, as Accipenser and Salmo, the vessels spring- 

 ing from the aortic system, and consequently conveying 

 arterial blood to the swimming-bladder, break up into a 

 wide-meshed capillary plexus ministering to the nutrition of 

 the organ, from whence the blood passes into systemic veins ; 

 in many others, however, peculiar vascular arrangements are 

 found in the external part of the internal layer, which were 

 first carefully examined by Joh. Miiller, and referred by him 



Fig. 137. 



Fig. 137. Epithelium detached from the internal surface 6f an 

 alveolus of Testudo groeca. Magnified 200 dia'meters. 



to the category of retia mirabilia. In these instances arteries 

 suddenly break up into sometimes expanded, sometimes more 

 restricted, brushes or tufts of vessels, from which tree-like 

 plexuses of capillaries originate either immediately or after 

 the formation of a few larger vessels. From these plexuses of 

 capillaries distributed upon the internal surface of the 

 swimming-bladder, venous retia mirabilia take origin (either 

 directly or after the formation of a few large veins), which are 



* Leydig describes a layer of smooth muscular fibres in the external 

 layer of Esox, but I have not been able to discover it. 



VOL. II. G 



