80 THE LUNGS, BY FRANZ EILHARD SCHULZE. 



easily detached. The thick map-like layer of the swimming- 

 bladder of the sturgeon presents quite a peculiar connective- 

 tissue formation ; for, putting aside a sparing quantity of loose 

 fibrillar stroma, it is entirely composed of relatively short 

 fusiform and flattened fibres, which, on the one hand, unite to 

 form larger bands, and on the other can be broken up into 

 smaller elements, presenting similar characters. In addition 

 to small, short, dark, longitudinal strise, which may be regarded 

 as the indications of connective-tissue corpuscles, no structure 

 can be distinguished in these highly refractile and, as I here 

 particularly desire to state, strongly doubly refracting elements.* 

 On boiling, and after treatment with acids, they swell up to an 

 extraordinary extent, and quickly dissolving form gelatine. 



Muscular tissue, sometimes of the striated, and sometimes of 

 the smooth form, is associated with the connective- tissue 

 matrix of the swimming-bladder in very various modes. The 

 swimming-bladder of Polypterus bichir and Amia is invested 

 immediately beneath the peritoneum by a sheath consisting 

 of two superimposed and decussating layers of transversely 

 striated muscular tissue. In the latter each layer is composed 

 of only a single layer of fibres, but in Polypterus there are a 

 considerable number of layers in each. In a third Ganoid, 

 Lepidosteus osseus, bands of transversely striated muscular 

 fibres, which either anastomose with one another directly 

 or through the intermediation of tendinous cords into a 

 plexus, lie, not on the external surface of the swimming-bladder, 

 but in the here largely developed membranous septa and 

 trabeculse that surround the alveoli of the internal surface. In 

 the sturgeon, on the other hand, there is a continuous layer of 

 smooth muscular tissue in the external fibrous layer. A few 

 osseous fishes, as, for example, Trigla, Dactyloptera, and Zeus, 

 possess sharply defined plates or bands of transversely striated 

 muscular tissue, situated on certain parts only of the external 

 surface of the swimming-bladder, others, as the Cyprinoids, have 

 in the middle line of the anterior portion of the internal layer, 

 and on its inferior aspect, a longitudinal band of transversely 



* The optic axis corresponds to the longitudinal axis of the fibres, 

 which, like muscular fibres, are positively doubly refracting. 



