26 SEGMENTAL COMTOSITION. 



stage of tissue change.^ In the sturgeon, skate, and shark, 

 it stops at the gristly stage, and hence these fishes are 

 called " cartilaginous." In most fishes, and all air-breath- 

 ing vertebrates, it proceeds to the bony stage, with the 

 subsequent modifications and developments above recited. 



The main part of the skeleton — what ma}^ be termed the 

 skeleton proper — consists of the neuro-skeleton ; and it 

 is in the construction of this system that the most interest- 

 ing and beautiful evidences of unity of plan, as well as 

 of adaptation to end, have been discerned. The parts of 

 the neuroskeleton are arranged in a series of segments, 

 following and articulating with each other, in the direc- 

 tion of the axis of the body, from before backwards in 

 brutes, from above downwards in man. 



Each complete segment, called " vertebra," consists of 

 a series of osseous pieces, arranged according to one and 

 the same plan (Fig. 3), viz : so as to form a bony hoop, or 

 arch, above a central piece, for the protection of a seg- 

 ment of the nervous axis, and a bony hoop, or arch, 

 beneath the central piece, for the protection of a segment 

 of the vascular system. The upper hoop is called the 

 " neural arch," N. (Gr. 7ieuron^ nerve) ; the lower one, the 

 "haemal arch," H (Gr. haima, blood) ; their common centre 

 is termed the "centrum," c (Gr. kentron, centre). The 

 neural arch is formed by a pair of bones, called "neura- 

 pophyses," n n (Gr. for nerve and apophysis, a projecting 

 part or process) ;' and by a bone, sometimes cleft or bifid, 

 called the "neural spine," ns; it also sometimes includes a 

 pair of bones, called " diapophyses," c? c? (Gr. dia, across, 

 or transverse, and apophysis). The h^mal arch is formed 



^ The doctrine or stndj of this kind of development — the development 

 of substance and texture, as contradistinguished from that of size and 

 shape — is now termed " Histology," from the Greek histos, net or tissue, 

 and logos, a doctrine or discourse. 



