THE SKELETON OF THE VERTEBRATE HEAD. 185 



The fundamental facts on which the preceding determina- 

 tion of the comparative constitution of the post-stomal neural 

 arches of the cranium depends, are the zygopophyseal con- 

 nections of the human " petrosals." If the " petrosals" even 

 in one species can be proved to present the characters of 

 neurapophyses, the sclerotome to which they belong must 

 exist in addition to those to which the "ali-sphenoids" and 

 " orbit o-sphenoids" are referable. The existence of temporal 

 neurapophyses explains the existence of interparietal, in 

 addition to parietal bones in the mammal ; both of these 

 meta-neurapophyses taking part in the protection of the 

 developed cerebrum ; while the non-appearance of the anterior 

 or spheno-parietal in the bird, reptile, and fish, accords with 

 the complete development of the posterior or temporo-parietal, 

 repressed in the former by the influence of the cerebrum, and 

 by the full development of the ethmoido-frontal. I base my 

 determination of the separate existence and reciprocal develop- 

 ment of ethmoido-frontals and spheno-frontals, of spheno- 

 parietals and temporo-parietals, not only on my analysis of 

 the bones themselves in the series, but also on the evident 

 reciprocal influence which the superimposed cerebral mass in 

 the mammal, and the bulky organs of sense and uncovered 

 sense-ganglions of the lower Vertebrata have on the cranial 

 neural arches. I believe also, that in this, as in other 

 departments of inquiry, we are apt to look for greater sim- 

 plicity and uniformity in details than actually exist. The 

 simplicity of natural law consists in the comprehensiveness 

 of its general principles. In tracing these principles into 

 details, the complexity is found to be infinite. 



The Hcemal Arches of the Post-stomal Cephalic Sclerotomes. 

 The clue by means of which we can alone be safely guided 

 to the morphological constitution of these arches, in the midst 

 of the varied complexity which they present to the compara- 

 tive anatomist, is afforded by embryology. The hsemal arches 



