without possessing special muscles of its own, may, nevertheless, be partially invested by tendinous, or partly muscular and partly tendinous, extensions from the muscles of the body-wall (Fig. 208), or may be intimately related to certain muscles connected with the pectoral girdle. {360}[Illustration: FIG. 207.—Ventral view of the air-bladder and its extrinsic muscle in _Platystoma_. _a.b_, Air-bladder; _a.l.c_, left antero-lateral caecum of the bladder; _b.o_, basioccipital; _b.w_, body-wall in contact with the lateral wall of the bladder; _c^1_, centrum of the first vertebra; _cl_, clavicle; _d.p_, ductus pneumaticus; _m^1_ and _m^2_, extrinsic muscles of the bladder; _pt.i_, post-temporal. (From Bridge and Haddon.)] [Illustration: FIG. 208.—Air-bladder and its muscles in _Micropogon undulatus_. _a.b_, Air-bladder; _l.b_, right lateral caecum; _m_, _m_, musculo-tendinous extensions from the muscles of the body-wall, which partially invest the surface of the air-bladder. (From Sörensen.)] Whatever the precise relation of the air-bladder to its muscles it is probable that the physiological effect is in most cases the same. By the rapid alternating contraction and relaxation of the muscles, some part of the wall of the bladder becomes alternately compressed and relaxed in such a way as to initiate a series of vibratory movements in the gases of that organ, and so produce definite sounds. In not a few of the Fishes the cavity of the bladder is subdivided by external constrictions or by internal septa, or is complicated by the development of lateral, tubular, caecal branches; and hence the vibratory movements of the gases will be greatly strengthened by their passage across the edges of the septa, or the apertures of the caeca, and the intensity of the resultant sounds also increased. It will be readily understood that the nature and quality of the sounds emitted by different Fishes will necessarily vary with the shape of the air-bladder, the number and arrangement of the internal septa and the caeca, and the strength and disposition of the contracting muscles. In a few Teleosts (Triglidae and Zeidae) sounds are said to be produced by the rapid vibration of an annular, or centrally-perforated, muscular diaphragm, which stretches across the cavity of the air-bladder.[423] Nevertheless, it must be strongly emphasised that, while in some Fishes the air-bladder and its muscles {361}undoubtedly constitute a vocal organ, there are many others in which the bladder can only be inferred to be sound-producing from its general agreement in anatomical structure with the same organ in Fishes where its vocal function has been clearly proved.