842 



OSSEOUS SYSTEM. (CoMP. ANAT.) 



In all the mammiferous Quadrupeds pos- 

 sessed of unguiculate feet, the digital phalanges 

 of the anterior extremity present nothing worthy 

 of special notice in this place, minor differences 

 being noticed under the proper heads; but in 

 the ungulate Pachydermata, such as the Rumi- 

 nants and Solipeds, remarkable exceptions to 

 the usual arrangement are met with. In the 

 Ruminants only the two central fingers are well 

 developed, each consisting of three large pha- 

 langes, the distal one of which is enclosed in a 

 strong hoof, so as to give the cloven appearance 

 to the whole foot which is so characteristic of 

 the order; but besides these, two rudimentary 

 toes exist, one on the outer, the other on the 

 inner side of the foot, but so small as not to 

 reach the ground or to be serviceable in the or- 

 dinary progression of these creatures. 



In the Solipeds even the division between 

 the central large toes that exists in the Rumi- 

 nant becomes obliterated, and the whole foot 

 appears to be made up of a single toe consisting 

 of three strong phalanges, the distal one being 

 encased in a large semicircular hoof. Even in 

 these animals, however, rudiments of two other 

 toes are distinguishable, but very imperfectly 

 developed. 



Ilium. This is the principal bone entering 

 into the composition of the pelvic arch, and fre- 

 quently is the only one met with in this part of 

 the skeleton. In the osseous Fishes it is not yet 

 connected with the spine, so that the posterior 

 part of the body is left perfectly free and un- 

 trammelled, in order to allow of the extensive 

 movements of the tail required for the propul- 

 sion of these aquatic animals through the water. 

 There is consequently here no sacrum, and we 

 are not surprised to see the posterior limbs ex- 

 tremely variable in their arrangement, being 

 placed far back or advanced towards the ante- 

 rior part of the body as circumstances require. 

 Even in the cartilaginous Fishes the pelvis has 

 no connection with the spine, the whole con- 

 sisting of a broad transverse osseous band 

 placed beneath the terminal portion of the 

 abdomen. 



In the Batrachia, too, the iliac bones retain 

 to some extent the form of ribs, and in the 

 Frog are two bones of considerable length 

 attached to the prolonged transverse processes 

 of the last vertebra, which of course, in this 

 case, represents the sacrum. 



In the Toad another step is made towards 

 strengthening the posterior part of the spine, 

 preparing it to support locomotive organs of 

 greater energy by fixing the iliac bones to two 

 of the vertebral transverse processes, forming a 

 sacrum composed of two bones, which is in 

 fact the usual condition of this part of the ske- 

 leton in the higher Reptiles. 



In the Chelomans, however, the iliac bones 

 are again attached to asingle vertebra and pelvis, 

 like the bones of the shoulder placed internal 

 to the ribs which form the carapax or dorsal 

 shield. 



The ilium, in all the class of Birds, is enor- 

 mously developed in proportion to the unfa- 

 vourable circumstances under which they sup- 



port themselves upon their posterior extre- 

 mities. It extends along the sides of the ver- 

 tebral column, to which it is solidly anchy- 

 losed, converting into one immense sacrum 

 from eight to nineteen of the posterior vertebra, 

 which are so completely fused to each other 

 and to the iliac bones that their number is only 

 distinguishable from the positions of the inter- 

 vertebral foramina through which the nerves 

 escape from the spinal canal in this region. 



In Mammals the iliac bones are likewise 

 greatly developed, except in theCetacea, where, 

 in consequence of the necessity for fish-like 

 flexibility in the hinder part of the body, no 

 hinder extremities exist. The sacrum is com- 

 posed of a considerable number of vertebrae 

 here anchylosed together and considerably mo- 

 dified in their form, to which the broad assa 

 ilii are firmly secured by ligaments and an 

 interposed cartilage, giving a firmness to this 

 part of the skeleton second only to what is 

 observable in the feathered races. 



The ossa iscliii, the second elements entering 

 into the composition of the pelvic framework, 

 are not so invariably present as the iliac bones. 

 In Fishes they are not to be found : but in all 

 the Reptilia, where the elements of the skeleton 

 remain permanently disunited to a much greater 

 extent than in warm-blooded animals, they are 

 constantly present, except, of course, where the 

 hinder extremities are deficient, and are sepa- 

 rated by a very distinct line of demarcation 

 from the other bones of the pelvis. 



As in the shoulder, the articular cavity for 

 the attachment of the anterior limbs when all 

 the elements of that part are fully developed, is 

 formed by the union of three bones, so likewise 

 in the pelvis, which is only a repetition of the 

 same apparatus modified in form, do all the 

 three bones of which it consists enter into the 

 formation of the cotyloid cavity, a circumstance 

 which, in Reptiles, is particularly conspicuous. 



In the class Aves, notwithstanding the aber- 

 rant condition of the pelvis, the ischia are easily 

 distinguishable from their position, bounding 

 as they do the obturator foramen on the one 

 side, and the sacro-ischiatic notch on the other. 



In the Cetacean Mammals this element, of 

 the skeleton is again obliterated, but in all the 

 other orders it is present, and in the earlier 

 stages of life is readily demonstrable as a dis- 

 tinct bone of the pelvis. 



The ossu pubis are the third pair of elements 

 entering into the composition of the pelvic ca- 

 vity, and to these the same remarks are appli- 

 cable as we have already made concerning the 

 ischia. In Fishes they are not present, but 

 throughout the Reptile orders that possess a 

 pelvis they are very distinct and important parts 

 of the skeleton, meeting each other anteriorly 

 in the mesial line, where they are united by a 

 strong svmphysis. 



The pubic bones in Birds occupy a cor- 

 responding position ; they are here, however, 

 remarkable from the circumstance that their 

 distal extremities never (except in the Os- 

 trich) meet to form a pubic symphysis, but are 

 always widely separated from each other, an 



