ON THE VERTEBRATE SKELETON. 305 
_ of the divided spine (13), mainly characterize the neural arch (N rv) of the 
terminal or nasal segment of the human skull. The early coalescence of each 
heemapophysis(21) with the corresponding half of the divided hzemal spine (22), 
and the unusual expansion of the bones, especially the second (27), which 
diverge from the hemapophysis, form the chief characteristics of the hzemal 
arch (H 1v) of the nasal segment. The hemapophysial portions of both the 
nasal and frontal vertebre are much less elongated than in most other 
animals. 
It may serve to test the accuracy of the general homologies here assigned 
to the bones of the human skull, if we notice the degree to which they have 
been subject to modification in connection with such determinations. 
According to the general character of the vertebral elements in the rest of 
the frame, we should be prepared to expect that the hemal arches would be 
subject to a greater variety in respect of development and relative position 
to their segments than the neural arches; and that in the latter the parts 
determined as centrums and neurapophyses would retain more of the or- 
dinary proportions of such parts in other segments or in other animals, than 
the peripherally situated spines. If new bones are added, we should expect 
to find them in the relative position of appendages to the normal vertebral 
arches: or should these be homologous with similar superadditions in the 
skulls of lower animals, they will probably be the seat of more extensive 
changes of form, proportion and connections, than the elements of the verte- 
bral arches themselves. : 
Now if the reader will glance at fig. 25 and compare the bones forming 
the segments of the skull with those in figs. 24, 23, 22 and 5, he cannot but be 
struck with the remarkable degree of uniformity in the dimensions of the 
bones 2, 6 and 10: no. 14 being the terminal neurapophysis, has been the seat 
of mere variety ; but the general steadiness of this series of bones in regard 
to their dimensions and connections accords with the characters assigned to 
them, as-neurapophyses, which are always the most constant and important 
of the ossified vertebral elements. 
The bones 1, 5, 9 and 13 equally conform in the kind and degree of their 
modifications with their determination as the bodies of the vertebre. 
The increasing capacity of the neural canal of the head, demanded for the 
lodgment of the progressively expanded encephalonas the vertebral scale rises, 
is chiefly acquired by the expansion of the bones, 3, 7, 11, which, being deter- 
mined as ‘neural spines’ in the fish, might be expected to be subject to greater 
deviations from their typical form and proportions than the more central 
and essential parts of the neural arches. ‘The terminal neural spine, 15, is 
subject to still greater varieties in the range of species, as might also be ex- 
pected from its position. In one mammal, e.g. the porcupine, it is more 
expanded than any of its succeeding homotypes in the cranium; in man its 
proportions are so much reduced as greatly to mask the homotypal relation. 
Tn one mammal, e.g. the orang, the nasal spine is not only diminutive but 
single: in another mammal, e.g. the manatee, it is also diminutive but di- 
vided, and the halves completely separated by the intervention of part of the 
succeeding spine. 
The abnormal conditions of the human skull give further illustration of the 
truth of these general homologies of the cranial bones, and reciprocally re- 
ceive light from such determinations. In the case of idiots from defective 
growth or development of the brain, where the cavity of the cranium is re- 
duced to half or less than half its normal capacity, as e. g. in the skull described 
and figured in my ‘ Memoir on the Osteology of the Chimpanzee*,’ it might 
* Zoological Transactions, vol. i. p. 343, pl. 57 and 58. 
