AMPHIBIA. 179 



may be conveniently termed axial, while those supporting the 

 limbs are appendicular. 



(1) The axial endoskeleton consists of the skull, backbone or 

 vertebral column, and breastbone or sternum. 



(a) The groundwork of the skull is made up of cartilage, 

 constituting the chondrocranium or primordial cranium. With 

 this are connected bones of two kinds, named, from their mode 

 of development, cartilage-bones and membrane-bones. The former 

 replace pre-existing cartilage, the latter pre-existing connective 

 tissue. It is convenient to consider the skull under the separate 

 headings of cranium, sense-capsules, jaws, and hyoid apparatus. 

 The membrane bones are flattened structures investing the other 

 parts, and will be taken last in each section. 



The cranium or brain-case is a narrow cartilaginous tube, the 

 upper side of which is broader than the lower, and possesses three 

 spaces, fontanelles, a larger anterior and two smaller posterior, 

 where cartilage is wanting. These are filled in by connective 

 tissue. The cavity of the brain-case opens behind by the large 

 foramen magnum. The region around this is known as occipital, 

 either side of which is occupied by an ex-occipital bone, upon 

 which is a projection, or occipital condyle, with an oval smooth 

 surface. The two ex-occipitals do not completely bound the 

 foramen magnum, a small cartilaginous strip being left above and 

 below. Each of these bones is perforated by a vagus foramen 

 through which the vasjus and glossopharyngeal nerves leave the 

 skull. 



The cartilaginous side-wall of the brain-case is perforated by 

 an optic foramen, and smaller foramina are also present. The 

 cartilage forming the front end of the brain-case is replaced by a 

 bony ring, the sphenethmoid or girdle bone, which also supports 

 the hinder part of the olfactory capsules that lie in front of the 

 skull. The sphenethmoid somewhat resembles a dice-box in 

 shape, and its cavity is divided into two halves by a transverse 

 partition marking the anterior boundary of the brain-case, and is 

 perforated by an olfactory foramen on each side for the passage of 

 the corresponding nerve. The front half of the bone is divided 

 by a vertical longitudinal partition, into right and left halves. 



Membrane Bones. The roof of the brain-case is invested by two 

 long, flat parieto-frontals, which are bent over behind so as to 

 protect the upper part of its side walls. The floor of the brain- 



