276 LECTURE XI. 



ganglion. This organ, however, is further composed of a number of 

 peculiarly arranged, elongated corpuscles, which depend in two rows 

 from each vascular branch, and are bound together by a loose cellular 

 tissue : the corpuscles are beset with fine villiform processes. The 

 blood returns from the vaso-ganglions by small veins which rarely 

 accompany, more commonly cross, the arteries. 4th. The two chief 

 ' retia mirabilia,' or vaso-ganglions, in the air-bladder of the Eel and 

 Conger, which are situated at the sides of the opening of the air- 

 duct, are also ' bipolar,' and consist of both arterioles and venules : 

 their efferent trunks do not ramify in the immediate margin of the 

 vaso-ganglion from which they issue, as in the vaso-ganglions of the 

 Cod, Bui'bot, Acerine, and Perch, but run for some distance before 

 they again ramify to form the common capillary system of the lining 

 membrane of the air-bladder. Rathke * failed to detect the open- 

 ing of the air-duct with the oesophagus in the Eel ; but De la 

 Roche had well described the oblique aperture "j", and accurately 

 cites the whole family of the Eels as fishes having both the so- 

 called ' air-gland ' and the pneumatic duct. It had been supposed 

 that the vascular 'air -gland' was present only in those fishes which 

 could not derive the gazeous contents of their swim-bladder from 

 without ; and unquestionably in those fishes which have the shortest 

 and widest ducts (Sturgeon, Amia, Erythrinus, Lepidosteus, Lepi- 

 dosiren, Polypterus), the supposed air-secreting vaso-ganglions are 

 not developed. Since Professor Magnus has determined the ex- 

 istence of free carbonic acid gas, of oxygen and of azote in the 

 blood, and dissolved in different proportions in the venous and 

 the arterial blood, it may be readily conceived, as Professor Mliller 

 well remarks if, that the venules of the vaso-ganglions may withdraw 

 carbonic acid gas from the arterioles, and that these may reach the 

 inner surface of the air-bladder richer in oxygen and poorer in car- 

 bonic acid than when they penetrated the vaso-ganglions. 



The air-duct may allow the gas to escape under certain circum- 

 stances ; and the small size and obliquity of its orifice in many 

 osseous fishes (Carp, Eel) seem only to adapt it to act as a safety- 

 valve against high pressure when the fish sinks to great depths, or 

 sudden expansion of the gas when they rise to the surface § : but 



* cix. ' Ueber die Schwhnm-blase einiger Fische,' p. 98. f cxv. p. 201. 



^ XXI. 1841, p. 98. See also Dr. J. Davy, in Phil. Trans. 1838. 



§ Neither the air-duct nor the elasticity of the air-bladder are equal to prevent 

 the consequences of a too rapid removal from the enormous pressure which fishes 

 sustain at great depths iti the sea : those that are drawn up quickly by the hook 

 are often found to have the air-bladder ruptured, and sometimes the stomach is 

 protruded from the mouth by the pressure of the suddenly extricated and expanded 



