9 8 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The chondrocranium of the MAMMALS has several peculiarities. There are 

 four occipital vertebrae, the last only with a complete vertebral character, all event- 

 ually fusing with the synotic tectum. The dorsal part of the otic capsule chondrifies 

 first, owing to the late development of the cochlear part of the ear in the lower half; 

 and the capsules themselves have their axes inclined, so that the exit of the seventh 

 nerve is on the anterior rather than on the lateral face. The trabecute soon join 

 the basal plate and from their sellar part an alary process is given off on either side 



Isr 



aop 



end 



FIG. 102. Chondrocranium of a pig, after Mead, as, alisphenoid ; cl, posterior clinoid 

 process; cr, fenestra cribrosa; end, foramen for endolymph duct;/w, foramen magnum; h, 

 fossa hypophyseos; Isr, lateral superior recess: os, orbitosphenoid; pi, parietal lamina; sn, 

 septum nasi; tn, tectum nasi; 2-12, exits of nerves. 



which extends upward to join an alisphenoid (ala temporalis) which chondrifies 

 separately, but soon joins the otic capsule above, leaving between them the foramen 

 ovale for the third branch of the fifth nerve, the other branches passing forward 

 over the ala and then between it and the orbitosphenoid (ala orbitalis) through 

 the sphenoidal fissure (foramen lacerum anterior). The ala orbitalis joins the 

 trabecula by two processes, bar and processes sometimes forming a reduced inter- 

 orbital septum. Later a marginal band (taenia marginalis) extends back from 



