THE CAKPUS AND THE TARSUS. 3i 



its occipital region are comparatively little affected ; but, in the 

 interorbital region, the frontal bones and the subjacent carti- 

 laginous, or membranous, side- walls of the cranium are thrown 

 over to one side ; and, frequently, undergo a flexure, so that 

 they become convex toward that side, and concave in the op- 

 posite direction. The prefontal 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 en- 

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

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

 forward, just as if this bridge represented the morphological 

 middle of the skull. (Fig. 10.) 



The embryonic PleuroneatidcB have the eyes in their nor- 

 mal places, upon opposite sides of tlie head ; and the cranial 

 distortion commences only after the fish are hatched. 



The Appendicular Endoskeleton.—The limbs of all verte- 

 brated animals make their appearance as buds on each side of 

 the body. In all but fishes, these buds become divided by 

 constrictions into three segments. Of these, the proximal is 

 called hracldum in the fore-limbs, femur in the hind ; the 

 middle is antebrachium, or crus ; the distal is tnanus, or pes. 

 Each of these divisions has its proper skeleton, composed of 

 cartilage and bone. The proximal division, normally, con- 

 tains only one bone, os humeri, or humerus, in the bracbium, 

 and OS femoris, ov femur, 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 longi- 

 tudinal series, except in the Ichthyosauria, where marginal 

 bones are added, and some of the digits bifurcate. 



The skeletal elements of the manus and pes are divisible 

 into a proximal set, constituting the carpus or tarsus ; and a 

 distal set, the digits, of which there are normally five, articu- 

 lated with the distal bones of the carpus and tarsus. Each 

 digit has a proximal basi-digital {metacarpal or metatarsal) 

 bone, upon which follows a linear series of phalanges. It is 

 convenient always to count the digits in the same way, com- 

 mencing from the radial or tibial side. Thus, the thumb is 

 the first digit of the hand in man ; and the great-toe the first 

 digit of the foot. Adopting this system, the digits may be 

 represented by the numbers i, ii, iii, iv, v. 



There is reason to believe that, when least modified, the 

 carpus and the tarsus are composed of skeletal elements 

 which are alike in number and in arrangement. One of these, 

 primitively situated in the centre of the carpus or ta,rsus, is 



