292 THE RABBIT 



Fourth Day 

 The Skeletal System. 



Remove the muscles from the head, neck, tiiink and limbs so as to 

 expose as much as possible of the skeletal system ; or, alternatively, examine 

 a prepared skeleton of the Rabbit. 



I. The Axial Skeleton. 



A. The Skull. 



The original cartilaginous skull (the chondrocranium) is almost entirely 

 replaced by bones. The skull therefore consists of these substitution or replace- 

 ment bones (also called cartilage bones) and of additional membrane bones, 

 i.e., bones of dermal origin which cover or invest the original cartilage. The 

 membrane bones are here marked *. 



The skull is composed of the cranium proper (the brain-case), the olfactorjy 

 and auditory capsules, the mandible or lower jaw, and the hyoid bone in the 

 floor of the mouth between the rami of the mandible. 



The cranium consists of segments (groups of bones) which come apart more 

 or less readily. 



Detach the skull from the atlas vertebra of the neck and note the following 

 parts : — 



(a) The occipital segment, or back of the skull : — 



(i) The basi-occipital, forming the floor of the hinder end of the 

 cranium and bounding the foramen magnum ventrally or in 

 front. 



(ii) The ex-occipitals, bounding the foramen magnum at the sides 

 and forming the greater part of the two condyles. Note the 

 par-occipital processes, directed backwards and closely 

 applied to the tympanic bulla. 



(iii) The supra-occipital, bounding the foramen magnum posteriorly 

 or above. 



(b) The parietal segment, which is connected with the occipital segment 



above and below, and separated from it at the sides by the auditory 

 capsules and squamosals : — 



(i) The basi-sphenoid, in front of the basi-occipital. 



(ii) The ali-sphenoids, fused to the sides of the basi-sphenoid and 

 forming part of the floor of the cranium. From the side of 

 each the external pterygoid process projects downwards, and 

 is connected along its inner edge with the palatine bone. 



(iii) The parietals,* meeting together mid-dorsally (sagittal suture) 

 and forming the roof of the cranium. 



(iv) The small median interparietal,* between the parietals and the 

 supra-occipital. 



(c) The frontal segment : — 



(i) The pre-sphenoid, a small laterally compressed bone in front of 

 the basi-sphenoid. 



