824 



OSSEOUS SYSTEM. (COMP. ANAT.) 



Fig. 433. 



Elements of a verfebra (after Owen). 



encrustment of bone as to resemble a single 

 mass ; and in both the Rays and Sharks there 

 are many more laminae enclosing the spinal 

 canal than there are bodies of vertebrae, bony 

 plates being developed over the junctions of 

 vertebral centres with each other as well as in 

 the usual situation, a circumstance which might 

 at first sight seem to militate against the views 

 adopted by modern physiologists concerning 

 the elemental constitution of this part of the 

 body, but from which, in reality, no legitimate 

 inference is deducible, seeing the extremely 

 confused and incomplete progress of ossification 

 in all the cartilaginous Fishes. 



Advancing to the osseous Fishes, such con- 

 fusion no longer exists, and every vertebra 

 assumes a precise form corresponding with the 

 particular uses assigned to it in the region 

 which it occupies. Before, however, proceed- 

 ing further, it behoves us to resolve an isolated 

 vertebra into the primary elements of which it 

 may itself be made up, and then we. shall 

 understand how all the varieties of shape pre- 

 sented by these bones are easily obtainable by 

 the simple exaggeration, diminution, or suppres- 

 sion of some of the elements composing it. 



Geoffrey St. Hilaire was the first anatomist 

 who pointed out the importance of thus analysing 

 the different portions of the osseous system, 

 and the views which were promulgated by that 

 learned writer were generally adopted until 

 Professor Owen, in the course of his researches 

 concerning the composition of the skeletons of 

 extinct British Reptiles, was led, as we think 

 very justly, to modify considerably the views 

 which had been previously entertained upon this 



subject; we cannot therefore do better than lay 

 before the reader the conclusions deduced by 

 Professor Owen from a very elaborate and exten- 

 sive survey of the various forms of the skeleton. 



" A vertebra," says Professor Owen, " may 

 be traced through its various degrees of com- 

 plication, either during the progressive stages 

 of its developement, or by taking permanently- 

 formed vertebrae of different grades of com- 

 plexity in different animals; or in many instances 

 by comparing the vertebrae in different parts of 

 the spine in the same animal." 



The terminal vertebrae of the tail in most 

 species exhibit the simplest condition of these 

 bones. The most complicated vertebra are 

 those of the lower part of the neck of certain 

 birds, as the Pelican, or at the beginning of 

 the tail of a Python or other large Serpent. 



The parts or processes of such a vertebra 

 may be divided into autogenous, or those which 

 are independently developed in separate carti- 

 lages, and exogenous, or those which shoot out 

 as continuations from these independent con- 

 stituents. The autogenous or true elements are 



1. The centrum or body of the vertebra 

 (Jig. 433, rf,) which in Mammalia, as Cuvier 

 has observed, is complicated by two epiphyses. 



2. Two superior laminae (l>, 6) developed to 

 protect the great nervous cord which rests on 

 the upper surface of the centrum, and which 

 Professor Owen therefore proposes to call 

 Neurapophyses. 



3. Two inferior laminae (e, e) developed 

 generally to protect the great bloodvessels on 

 the under surface of the centrum, and which 

 may be called Htemapophyses. 



4. The superior spinous process (a) which is 

 connected and generally anchylosed with the 

 distal extremities of the neurapophyses, and 

 forms, in conjunction with those processes, the 

 superior arch of the vertebra. 



5th. An inferior spinous process which is 

 connected and commonly anchylosed with the 

 distal extremities of the Hamapophyses, form- 

 ing in conjunction with these a chevron or 

 V-shaped bone. 



To the category of autogenous vertebral 

 pieces likewise belong the ribs (cc), which 

 are generally anchylosed to the other vertebral 

 elements in the cervical, sacral, and caudal 

 vertebrae of the warm-blooded Vertebrate classes. 



The propriety of regarding the ribs as verte- 

 bral elements is well illustrated in the Plesio- 

 saurus, in the cervical, sacral, and caudal 

 vertebrae of which they have been generally 

 described as transverse processes, although they 

 are separate bones. 



True transverse processes are always exoge- 

 nous, or mere projections from the centrum or 

 the neurapophyses, and are of secondary impor- 

 tance. They are of two kinds, superior and 

 inferior ; both are present in the cervical ver- 

 tebrae in most classes of the vertebrated animals; 

 the inferior transverse processes alone are deve- 

 loped in Fishes. 



The oblique or articulating processes are also 

 exogenous, and may be developed either from 

 the neurapophyses or the base of the superior 

 spines of the vertebrae. 



As in other complicated bones resulting from 



