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MANUAL OF ZOOLOGY 



thoracic region are characterised by the possession of ribs, 

 which, in the case of the most anterior, are connected with 

 the breast-bone or sternum by slender cartilaginous sternal 

 ribs. In the lumbar region there are no ribs. The sacral 

 region is distinguished by its relations with the hind limb. 

 The caudal region, short in the rabbit, very long in the 

 lizard, lies behind the sacral. The ribs connected with the 

 thoracic vertebrae are slender curved rods, which lie in 

 the side walls of the anterior part of the trunk ; the most 

 anterior of them with their continuations, the sternal ribs, 

 form half-loops extending from the spinal column dorsally 



cent 



Fig. 2ii. - LepUS cuniculus. A, atlas and axis, ventral aspect; od, odontoid pro- 

 cess of axis. B, lateral view of axis; art, articular facet for occipital condyle; 

 od, odontoid process; pt. zy, post-zygapophysis; sp, neural spine. C, thoracic 

 vertebrae, lateral view, cent, centrum; fac, facet for rib; t)iet, metapophysis; 

 pr. zy, pre-zygapophysis : pt. zy, post-zygapophysis ; rb, rib ; sp, spinous process. 



to the sternum ventrally. The sternum or breast-bone, 

 absent in the dogfish, lies in the middle of the wall of the 

 ventral region of the trunk. In the lizard it is a rhomboidal 

 plate of cartilage ; in the rabbit it is bony, and divided up 

 into a number of segments known as the sternebrce. 



In the embryo of each of the three forms used as illus- 

 trations, the spinal column passes through a stage in which 

 it consists merely of a continuous cylindrical rod of cells — 

 the notochord, corresponding to the notochord of Amphi- 

 oxus — which becomes enclosed in a sheath. In some 



