vi. ] GENERAL VIE IV OF INTERNAL SKELETON. 219 



Generally the axis of the skeleton (i.e. the backbone) con- 

 sists of a number of segments (vertebras) which are serially 

 homologous one with another. 



7. From this axis we have seen corresponding arches (carti- 

 laginous or osseous) to arise and enclose the spinal marrow, 

 which is the central part of the nervous system of the trunk. 

 They also are serial homologues (homotypes) one of another ; 

 and such arches, as they extend above the axis, have been 

 called epaxial ' arches. In man such arches are wanting (as 

 bony structures) in part of the sacral and in the coccygeal 

 regions ; yet we have seen how numerous and complete they 

 may be in those regions in other animals. 



8. We have seen also that a number of arcs may extend 

 out from the central axis on each side, and that these may 

 descend and meet in the middle line below, so as to form 

 a series of large ventral arches enclosing the body cavity. 

 They also are homotypes one of another ; and such arches, 

 as they extend from the sides of the backbone, have been 

 called par axial' 2 arches. In man such arches are complete 

 only in the thorax, where the lateral arcs, each composed of 

 a rib and cartilage, are medianly united by the help of the 

 sternum. 



It is evident, however, that these arcs may be much more 

 extensively developed in regions where they are wanting in 

 man, as e.g. in the cervical vertebras of the Crocodile, and in 

 the anterior coccygeal vertebrae of some Chelonians and of 

 Menobranchus. 



9. Each paraxial arc, however, has been seen very gene- 

 rally to have two attachments to the backbone one above, 

 the other below, i.e. one more dorsal than the other. 



In man we have the transverse process and tubercle of the 

 rib placed dorsally ; and the surface for the head of the rib, 

 the head itself and the neck, more ventrally. In other cases, 

 e.g. Menobranchus^ we find a distinct dorsal (or tubercular) 

 process, and a more ventral (or capitular) process, giving 

 attachments respectively to the diverging branches of a 

 Y-shaped rib. 



Sometimes we have seen the ribs bifurcate distally as 

 well as proximally, as in Salamandra ; and sometimes, as in 

 certain Fishes, e.g. the Tunny and Polypterus, there are two 

 series of ribs on each side, one above the other. 



We may thus distinguish two series of paraxial parts 

 on each side, one made up of tubercular processes (or 



1 'ETTI, upon (the skeletal axis). 2 Flapa, beside (the axis). 



