844 



OSSEOUS SYSTEM. (CoMp. ANAT.) 



toes are more or less numerous. In their 

 most normal state of developement these bones 

 are five in number as in the human skeleton, 

 but from this variations occur in almost every 

 order of Vertebrata. 



The metatarsus of Reptiles is, however, well 

 developed, consisting of a series of moderately 

 elongated bones extended between the carpus 

 and the proximal phalanx of the corresponding 

 toe, but offering nothing worthy of special 

 comment. 



The tarso-metatarsal bone of Birds, repre- 

 senting both the tarsal and metatarsal portions 

 of the skeleton, seems to consist of three and 

 sometimes four metatarsal bones consolidated 

 into one piece. These are distinguishable in- 

 feriorly by the four trochlear surfaces that sup- 

 port the moveable toes ; while the presence of 

 an ossified spur in some gallinaceous birds, 

 regarded by many anatomists as the rudiment 

 of a fifth toe, might indicate the existence of a 

 fifth metatarsal element lost in the general 

 consolidation of these pieces. 



In all the unguiculate Mammals the meta- 

 tarsal bones hold the same relations with the 

 other bones of the foot as in the skeleton of 

 Man, and need no special notice; but in the 

 Ungulate families their appearance and arrange- 

 ment are necessarily much changed. In the 

 Solipeds and Ruminants the metatarsal division 

 of the extremity is so much elongated as to 

 constitute a very considerable portion of the 

 limb. It is principally made up of a single 

 piece generally called the " canon bone," 

 which in reality consists of two enormous meta- 

 tarsal bones consolidated into one, being fused 

 together in the central line along their whole 

 length, although the real composition of the 

 canon bone is always distinguishable both on 

 account of a deep furrow which indicates the 

 union of the two pieces, and from the con- 

 dition of the two widely separated trochlear 

 surfaces at its distal extremity. Besides the 

 two largely developed pieces forming the canon 

 bone, two other metatarsal pieces assist in form- 

 ing the foot of a Ruminating Quadruped ; these 

 sustain the supplementary toes developed in a 

 rudimental condition on the outer and inner 

 aspects of the member. 



The digital p/ui/tinges of the posterior ex- 

 tremity are among the most variable elements 

 of the skeleton, being, like those of the anterior, 

 made subservient to a great variety of uses both 

 in terrestrial and aquatic forms of Vertebrata. 



In the osseous Fishes they are represented 

 by the fine rays of the ventral fin, and are of 

 course employed in natation ; but in the carti- 

 laginous Fishes, as the Sharks and Rays, 

 although the resemblance between this part of 

 the skeleton and the feet of higher animals is 

 more striking than in the osseous races, they 

 are appropriated to a different office, serving 

 the purpose of claspers, whereby the intercourse 

 between the sexes is facilitated. 



Throughout all the Reptilia that possess 

 hind feet, the phalanges of the toes offer 

 nothing remarkable ; neither in Birds is there 

 anything peculiar in their structure, the only 

 circumstances of interest connected with this 



part of the skeleton in the feathered races 

 relating to the number and disposition of the 

 toes, and the presence of more or less numerous 

 joints entering into their composition. 



In the Mammalia this part of the foot cor- 

 responds in its composition with that of the 

 hand, and therefore need not be further noticed. 



In enumerating the elements of the endo- 

 skeleton it would be improper to omit certain 

 supplementary pieces, which, though not strictly 

 belonging to the osseous system, are of important 

 mechanical assistance to the muscles inserted 

 into different portions of the skeleton. These 

 are developed in the substance of various 

 tendons where much friction is encountered, or 

 where it is of importance to remove the line of 

 traction to some distance from the centre of 

 motion in order to gain additional leverage. 

 When developed in the tendons of the fingers 

 or toes, these detached pieces of osseous sub- 

 stance are called " sesamoid bones," but in 

 such situations their existence is by no means 

 constant. Connected with the ereat joints 

 corresponding with the knee and elbow of the 

 human subject, bones of this kind are very 

 generally developed, and their size and im- 

 portance renders them worthy of special remark. 

 In the anterior extremity the superadded bones 

 are named " olecranon," and very generally are 

 found solidly cemented to the proximal end of 

 the ulna, forming a prominent process, thatgives 

 great mechanical advantage to the extensor 

 muscles of the forearm. The corresponding 

 bone appended to the knee-joint has, from its 

 condition in the human subject, received the 

 name of" patella." 



Such being the elements employed by 

 nature in constructing the locomotive extre- 

 mities of Vertebrate animals, our only won- 

 der is that by simply modifying the figures 

 of the bones, by suppressing some and 

 exaggerating others, or else by fusing several 

 of them together, such infinite diversity of 

 apparatus is provided in the various classes of 

 Vertebrata. Seldom, indeed, does a limb 

 present all the pieces we have enumerated in a 

 complete state of developement, and frequently 

 the majority of them, or even the whole series, 

 is entirely dispensed with. In many Fishes, 

 as in the Lamprey and Myxine, all four ex- 

 tremities are absolutely wanting, a circumstance 

 which again becomes remarkable in the case of 

 Ophidian Reptiles. Occasionally the hinder 

 only are called into existence, and that in a 

 very rudimentary state, as for example, in some 

 Serpents, Anguis, Boa, &c. More frequently 

 the anterior limbs are found without the pos- 

 terior ; such is the case in the apodal Fishes, in 

 the Siren and Bimanes among Reptiles, and 

 still more conspicuously in the Cetaeea. 



EXOSKELETON. Having thus examined all 

 the elements that belong properly to the osseous 

 system or endo-skelcton, we must now turn 

 our attention to another important system of 

 organs equally concerned in building up the 

 framework of the body, and that to an extent 

 which the human osteologist would scarcely 

 imagine possible. \Ve allude to the external 

 or cuticular skeleton, which, although reduced 



