ON THE VERTEBRATE SKELETON. 305 



of the divided spine (13), mainly characterize the neural arch (N iv) of the 

 terminal or nasal segment of the human skull. The early coalescence of each 

 hsemapophysis(2i)with the corresponding half of the divided haemal spine (22), 

 and the unusual expansion of the bones, especially the second (27), which 

 diverge from the haemapophysis, form the chief characteristics of the haemal 

 arch (H iv) of the nasal segment. The haemapophysial portions of both the 

 nasal and frontal vertebrae 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 haemal 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 io: no. 14 being the terminal neurapophysis, has been the seat 

 of more 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, s, 9 and 13 equally conform in the kind and degree of their 

 modifications with their determination as the bodies of the vertebrae. 



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. 

 In 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, pi. 57 and 58. 



