. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 290 RESPIRATION. gill-bearing arches are not composed of single undivided curved bones, but of several ele- ments, adjusted with express reference to the elasticity and flexibility of the whole. Six of these arches are primarily developed, and five permanently retained. The first four support gills, the fifth is beset with teeth which guard the opening of the gullet: this latter is termed the pharyngeal arch; the rest the branchial arches. From the convex side of the branchial arches a double series of interlocking penkn
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 290 RESPIRATION. gill-bearing arches are not composed of single undivided curved bones, but of several ele- ments, adjusted with express reference to the elasticity and flexibility of the whole. Six of these arches are primarily developed, and five permanently retained. The first four support gills, the fifth is beset with teeth which guard the opening of the gullet: this latter is termed the pharyngeal arch; the rest the branchial arches. From the convex side of the branchial arches a double series of interlocking penknife-shaped processes radiate. On the Hat surfaces of these pro- cesses a gorgeous arrangement of mem- branous leaflets is disposed in a transverse manner, each leaflet standing, as already de- scribed, on its edge (jig. 237.). This rich foliage, bearing the complex web of the respiratory capillaries, is itself sustained by a machinery of elastic solid parts, hitherto unknown in comparative anatomy. The series of curvilinear bones denomin- nated the arches of the branchiae, and exhi- bited in section at a, fig 237., are inferiorly attached to the sternal chain of bones, pro- ceeding upwards and backwards, and describ- ing a curve, which in different genera varies in decrees of sharpness, and which finally affix themselves by means of ligaments to the base of the cranial bones. These curved bones are constructed of several separable pieces, adjusted with artful reference to the resilient properties of the curvilinear figure. The act of opening the mouth in the fish deter- mines a consecutive series of movements, which end in the preparation of the gills to be traversed by the branchial current. By an appropriate intervening mechanism the move- ments of protruding and retracting the mouth occasion irrespectively the approximation and separation of the branchial processes, accom- panied by an alternate increase and decrease of the curvature of the sustaining arch. The straight pen
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