36 THE FROG 
dorsal surface between the skull and the neural arch of the atlas. 
In life this gap is closed by a strong membrane, but it affords a 
convenient place for beginning dissection to expose the central 
nervous system, and for dividing and destroying the central 
nervous system in the operation known as “ pithing.”’ 
The second to fourth vertebre have unusually strong trans- 
verse processes; why? The ninth vertebra (sacrum) has espe- 
cially strong backwardly directed transverse processes; what for? 
(c) The coccyx or so-called urostyle is a long cylinder of bone 
with a dorsal keel; it articulates in front, by two surfaces, with 
the body of the sacral vertebra. The neural canal is continued 
into the anterior portion of the coccyx. On the sides of the 
coccyx, and about the length of a vertebra from its anterior end. 
are a pair of small holes through which the tenth pair of spinal 
nerves pass out; these holes correspond to intervertebral foram- 
ina. The entire coccyx represents a row of fused vertebre 
(compare the spinal column of a tailed amphibian). 
2. The skull consists of (@) an axial portion, the cranium, 
which encloses the brain; associated with this are two pairs of 
capsules enclosing sense organs, viz., the olfactory capsules form- 
ing an anterior prolongation of the cranium, and the auditory 
capsules forming lateral expansions of the posterior region of 
the cranium; (b) the jaws and the framework by which they are 
attached to the cranium, and (c) the hyoid apparatus, situated 
mainly in the floor of the mouth. 
(a) The cranium. (1) Cartilage bones of the cranium. 
The exoccipitals are a pair of bones which together bound the 
foramen magnum or posterior opening of the cranium, which is 
continuous with the neural canal of the spinal column. On their 
posterior surfaces they bear the occipital condyles, two oval 
convex processes which articulate with the first vertebra or atlas. 
Near each occipital condyle is a foramen by which the vagus 
nerve leaves the skull. 
The proodtics are two bones forming the bony part of the audi- 
tory capsules and lying laterally and anteriorly to the exoccipitals. 
Each forms part of the orbit of the eye, and on its antero-ventral 
face bears a large notch through which pass in life the fifth, sixth 
and seventh cranial nerves. 
