App. I. CONDITIONS OF VARIABILITY. 455 



*' orthognathism," " opisthognathism." As in the 

 neural, so in the hgemal arches, the parts become sub- 

 ject to variety as they recede from the centrum. The 

 palatal bones (pleurapophyses) show most constancy, 

 the maxillaries (heemapoj^hyses) the next degree, the 

 pre-maxillaries (haemal spines) the least constancy.* 



So, likewise, with regard to the centrums them- 

 selves, the terminal one or " vomer " is more variable 

 than those behind it. 



The tympanic (pleurapophysis) offers as few cha- 

 racteristics to the ethnologist, as does the palatine. 

 The malar bones and zygomatic arches — diverging, 

 as appendages, from the maxillary arch — are seats of 

 variety only inferior to the neural spines. The 

 pterygoid processes are almost, if not quite, as vari- 

 able as the malar bones. 



Accordingly, the variability or value of ethnolo- 

 gical admeasurements depends on the vertebral ele- 

 ments, or general homology, of the parts they may 

 happen to include. The length of the skull is more 

 constant than that of the cranium, in the entire 

 series of human races, because it includes the ver- 

 tebral centrums, whilst the other includes only neural 

 spines. Moreover, the parts that chiefly vary the 

 length of the skull are those behind the foramen 

 magnum, and those before the palatine bones. 



The dimension from the anterior border of the 

 foramen magnum to the fore part of the pre-sphe- 



* The range of variety in this vertebral element may be estimated by 

 the fact that all the ordinal characters of the class of birds derived from the 

 " rostrum " are furnished by modifications of the premaxillary and pre- 

 mandibular bones. 



