of the Vertehrate Sl-eJeton. 33 



sides of the brain. Tlicy arc called (in seqnencc from behind 

 forward) exoccipital, ali.splienoid, and orbitosphenoid, and have 

 the same sort of relation to the base-bones that the lateral 

 elements of the upper arch of a series of three vertebrse have 

 to the three centrums out of which they rise. In the vertebrse 

 the upper bones, called neurapophjses, enclose the neural 

 substance, meeting above it. In the skull they do not meet 

 above ; but just as with the lateral elements of the inferior 

 vertebral arch, in the transition from the true caudal region to 

 the preanal or visceral region osseous elements come to be 

 introduced between them in some animals, which did not 

 exist in the tail, so in the transition from the upper arches of 

 the vertebrae to the upper arches of the skull, enlarged to 

 cover the brain, a sequence of bones is introduced, to roof 

 over the cavity, to which there is nothing corresponding in 

 the vertebral region. These bones, counting from behind 

 forward, are named supraoccipital, parietal, and frontal. 

 And all the bones enumerated differ from those of ver- 

 tebrae in touchinG; each other throu«rhout their lateral mar^-ins 

 by sutures or overlap — a condition which in the vertebral 

 column is only met with exceptionally, as in the cervical re- 

 gion of the rays, pipe-fish, &c., and a part of the vertebral 

 column called the sacTum, in many land-animals. And these 

 bones touching each other throughout tlieir extent, enlarge 

 the cranial cavity much in the same way as a sea-urchin en- 

 larges its covering shell. In the human skull there is some- 

 thing more, however ; there are bones which have existence 

 in relation to the senses : such are some bones which come in 

 between the first and second segments of the skull, and are 

 connected in a more or less evident way with the ear ; they 

 have been named collectively the otic bones. Then, between 

 the second and third segments, though external to them, is 

 usually one bone or more, developed seemingly in relation to 

 the eye : the lachrymal (and, perha])S, the malar) is such a bone. 

 And in front of the brain there are bones which have relation 

 to the nasal functions, and are named generally the ethmoid 

 bones. In possessing these sets of bones the bony investing 

 girdles of the brain differ in plan from the investing girdles 

 of the spinal column. 



If, now, we ask why there should be three segments in this 

 bony box for the brain, and why not an indefinite number of 

 segments as in the vertebral column, and why the structure 

 of the skull should become simpler the higher we ascend in 

 nervous organization, so that the three segments become more 

 and more well defined, the answer is, that the division be- 



Ann. & Mag, N. Hist. Ser. 4. Vol.^. 3 



