50 CLASS xiv. 



a calcareous matter. In some bony fishes the labyrinth is extended 

 by a production which unites with that of the opposite side (sinus 

 impar), and afterwards, on the outside of the occiput on each side, 

 terminates in an atrium on the first vertebra. Here, by means 

 of a chain of small bones, it is brought in connexion with the 

 swimming-bladder 1 , so that by this the intensity of the vibrations 

 of sound may be augmented. In other osseous fishes the swimming- 

 bladder extends, without such a chain of bones, to membranous 

 spaces in the cranium which are in connexion with the auditory 

 apparatus 2 . 



The muscles of fishes consist of loosely united fibres, generally 

 white. On each side of the trunk a large muscular mass is 

 situated, which extends from the head and the osseous belt of the 

 pectoral fins to the base of the caudal fin, on the rays of which it 

 terminates by tendinous bands. This muscular mass is divided 

 by tendinous strips (ligamenta intermuscularia) , as though by ribs, 

 into segments lying behind one another ; the margins of these strips 

 appear on the surface under the skin as zigzag tendinous incisures 

 (inscriptiones tendinece) descending from the back to the abdomen. 

 This muscular mass is the lateral layer of the trunk-muscles, 

 of which in man the dorsal portion alone is present, and has been 

 developed to form the different muscles of the back. In fishes, on 

 the contrary, it extends over the abdomen. Where ventral fins 

 exist, the two lateral muscular masses separate from each other 

 to leave a fissure in which these fins are received; the pectoral 



1 These bones were named by E. H. WEBER, to whom we owe their discoveiy, Ge- 

 horknochelclien, ossicula auditus. GEOFFROY DE ST. HILAIRE, J. F. MECKEL and SAAG- 

 MANS MULDER refer them to the first vertebra; the largest, ensiform, hindmost bone, 

 which is immediately connected with the swimming-bladder, would seem to be the rib 

 of the second vertebra. See in detail the investigations of the author last named in 

 Bijdragen tot de NatuurJc. Wetensch. vi. 1831, bl. 84 105. According to OWEN these 

 bones, with the bony labyrinth, belong to the splanchnic skeleton. Lectures, n. p. 210. 



2 The first good description of the auditory organ of fishes was given by P. CAMPER 

 Verhandelingen van de Holl. Maatsch. der Wetensch. te Haarlem, vn. i, bl. 79 117, 

 with figs. 1 763. Great in this respect are the deserts of MONRO and SCARPA. This 

 subject has been treated with great completeness and detail by E. H. WEBER, in his 

 work De Aure et Auditu hominis et animalium. Cum tab. x. Lipsise, 1820, 4to. Comp. 

 also G. BRESCHET Hecherches anatotn. et physiol. sur I'Organe de Vouie des Poissons, avec 

 17 Planches. Paris, 1838, 4to, and J. MUELLER Ueber den eigenthumlichen Bau des 

 Gehororganes lei den Cyclostomen. Berlin, 1838. Mit 3 Kupfertafeln fol. (printed 

 separately from the Abhandl. der ATcad. der Wissensch. zu Berlin, aus dem Jahre 1837). 



