30 THE ANATOMY OF VERTEBEATED ANIMALS. 



The skull and face are usually symmetrical in reference 

 to a median vertical plane. But, in some Cetacea, the bones 

 about the region of the nose are unequally developed^ 

 and the skull becomes asymmetrical. In the Flatfishes 

 (Pleuronectidoe), the skull becomes so completely distorted 

 that the two eyes lie on one side of the body, which is, in 

 some cases, the left, and,' in others, the right side. In 

 certain of these fishes, the rest of the skull and facial bones, 

 the spine, and even the limbs, partake in this asymmetry. 

 The base of the skull and its occipital region are compara- 

 tively little afi'ected; but, in the interorbital region, the 

 frontal bones and the subjacent cartilaginous, or mem- 

 branous, side-walls of the cranium are thrown over to one 

 side ; and, frequently, undergo a flexure, so that they become 

 convex towards that side, and concave in the opposite di- 

 rection. The prefrontal bone of the side from which the 

 skull is twisted sends back a great process above the eye of 

 that side, which unites with the frontal bone, and thus in- 

 closes this eye in a complete bony orbit. It is along this 

 fronto-prefrontal bridge that the dorsal fin-rays are con- 

 tinued forwards, just as if this bridge represented the 

 morphological middle of the skull. (Fig. 10.) 



The embryonic Pleuronectidoe have the eyes in their 

 normal places, upon opposite sides of the head ; and the 

 cranial distortion commences only after the fish are hatched. 



The Appendicular Endosheleion. — The limbs of all verte- 

 brated animals make their appearance as buds on each side 

 of the body. In all but fishes, these biids become divided 

 by constrictions into thi'ee segments. Of these, the proximal 

 is called hrachium in the fore-limbs, femur in the hind ; 

 the middle is antebrachium, or cms ; the distal is inanus, or 

 pes. Ea<;h of these divisions has its proper skeleton, com- 

 posed of cartilage and bone. The proximal division, 

 normally, contains only one bone, os humeri, or humerus, 

 in the brachium — and os femoris, or fem,ur, in the thigh ; 

 the middle, two bones, side by side, radius and ulna, or 

 tibia and fibula ; the distal, many bones, so disposed as to 

 form not more than five longitudinal series, except in the 



