102 COMPARATIVE ANATOMY. 



became bent downwards, and thus the angle between it and the 

 median plane of the trunk was gradually reduced, until in Mammals 

 eventually, the longitudinal axis of the limb, when at rest, came to 

 lie parallel with the median plane of the body. In k the higher 

 types this is more particularly the case as regards tie posterior 

 extremities, the anterior undergoing the most varied adaptations 

 and modifications, and giving rise to tactile, prehensile, or flying 

 organs, or, as in aquatic Mammals, becoming once more converted 

 into rowing organs. 



Thus we may also reduce the limbs of all the higher Vertebrata 

 to a single ground-type, and we may further connect the latter 

 with the fin of Fishes by taking the ground-plan of the fin of 

 Ceratodus and Elasinobranchs, consisting of a main axis and 

 lateral rays, as a starting-point. Figs. 87 and 88 will render this 

 statement clear. In Fig. 88, a thick line (US) is seen beginning 

 at //, and passing through F t i, c, c, 2, to //. This is the main 



Pa 

 FIG. 87. DIAGRAMMATIC FIGURES TO SHOW THE RELATIONS OF THE FREE 



EXTREMITY TO THE TRUNK IN FISHES (A), AND THE HIGHER VETEBRATES (B). 



/S', pectoral arch ; Mt, metapterygium, which corresponds to the main ulnar ray 



( Ui) ; lid, secondary radial ray. 



axis, and from its proximal end (at II) a lateral ray passes off 

 through T, t, to /. A second series of lateral rays arises from 

 the other side of the axis. Thus we have here also the primitive 

 biserial form, with a marked preponderance of one row of radii. 

 At the same time it must be borne in mind that this arrange- 

 ment of radii on an axis is less plainly seen in the embryonic con- 

 dition than in the adult limb, and we must therefore speak of 

 the relations of these parts as similar rather than as strictly 

 homologous. 



The fore- and hind-limbs show a great similarity as regards the 

 form and position of their various parts. A division into four princi- 

 pal sections can always be recognised : in the case of the fore-liinb 

 these are spoken of as upper arm (brachium), fore-arm (anti- 

 brachium), wrist (carpus), and hand (manus); and in the 

 hind-limb as thigh (femur), shank (crus), ankle (tarsus), 

 and foot (pes). While the bone of the upper arm (humerus) 

 and of the thigh (femur), corresponding probably to the met a- 



