218 MODIFICATIONS OF THE HUMAN SKELETON 



pound bone gives a clue to tlie phenomena of its develop- 

 ment from so many separate points, which final causes 

 could never have satisfactorily afforded. As the centrum, 

 6, becomes confluent with ISTo. 1, a still more complex 

 whole results, which has accordingly been described as a 

 single bone, under the name of " os spheno-occipital" in 

 some anthropotomies. Such a bone has not fewer than 

 twelve distinct centres of ossification, corresponding with 

 as many distinct bones in the cold-blooded animals that 

 depart less from the vertebrate archetype. The spine of 

 the frontal vertebra (frontal bone) is much expanded and 

 bifid, like the parietal bone; but the two halves more fre- 

 quently coalesce into a single bone, with which the para- 

 pophysis (postfrontal) is connate. The pleurapophysis of 

 the haemal arch (tympanic bone) is reduced to its function 

 in relation to the organ of hearing, and becomes anchy- 

 losed to the petrosal, the squamosal, and the mastoid. 

 The htemapophysis is modified to form the dentigerous 

 lower jaAV, but articulates, as in other mammals, with a 

 diverging appendage (squamosal) of the antecedent hremal 

 arch, now interposed between it and its proper pleurapo- 

 physis; the two htemapophyses, moreover, become con- 

 fluent at their distal ends, forming the symphysis mandi- 

 bulce. 



The centrum of the first or nasal vertebra, like that of 

 the last vertebra in birds, is shaped like a ploughshare, 

 and is called "vomer;" the neurapophyses have been sub- 

 ject to similar compression, and are reduced to a pair of 

 vertical plates, which coalesce together, and with parts of 

 the olfactory capsules (upper and middle turbinals) form- 

 ing the compound bone called " aethmoid ;" of which the 

 neurapophyses (prefrontals) form the "lamina perpendicu- 

 laris" in human anatomy. The prefrontals assume this 



