362 On a Theory of the Skull and the Skeleton. 
the face, extending over the maxillaries, but entering, as in birds 
and Ichthyosaurs and most animals, into the anterior nares. In 
ruminants and pachyderms, where the pressure from the teeth 
is more uniform than in some animals, it is seen that the maxil- 
laries are deep and their upper and lower margins subparallel ; 
and, as though illustrating the community of origin, in some 
animals the palatines and pterygoids both bear teeth. The 
bones forming the elements of the oviparous lower jaw I believe 
to have been. developed as epiphyses of Meckel’s cartilage by 
pressure ; the dentary element presents the aspect of a terminal 
epiphysis, and the four other bones a superior and inferior and two 
lateral epiphyses, which functionally are a diapophysis. And now, 
of the important elements of the skull, there only remain the eyes 
and the ears, which correspond, in their relations to the ali- 
sphenoid, with the intervertebral nerves. The growth of the 
eye is a sufficiently evident cause of pressure to account for 
sclerotic, superorbital, and lachrymal bones; but the periotic 
bones, which have been so laboriously elaborated by Pro- 
fessor Huxley, appear to me to be nothing but ossifications 
around the auditory canals which have afterwards grown by 
contact with other ossifications. The quadrate bone is large 
when placed between the jaw and the skull, but dwindles to the 
incus when the pressure 1s removed; and so the mastoid, squa- 
mosal, and petrosal obviously owe their development to their rela- 
tions with the jaw. They are clearly sense-bones, and therefore, 
forming no part of the skull except as such, may be here passed 
over without further notice. 
Such, then, is an outline of the mechanical theory of the skull; 
and such are some of the chief points which I hope to illustrate 
and demonstrate in the collections of fossil vertebrata which are 
among the best treasures of the Woodwardian Museum. ‘This 
theory differs from others in the subordination of structure to func- 
tion, and the belief that, except for the variation in organization, 
similar functions will always develope similar structures. It differs 
from other theories in giving a mechanical reason for the presence 
of every bone. Its final conclusion is, that the skull is the terminal 
segment of the body, and that, just as the adjacent segments 
consist of the pharynx, the larynx, and a vertebra enclosing part 
of the neural column, so also the skull, which is the termina- 
tion of these three organs, and where their outlets are visible, 
must consist of them also; that the brain-case, therefore (the 
termination of the neural system), is a modified vertebra, that 
the bronchial circle of nasal and palatine bones is a modification 
of the trachea, and that the lower jaw is a modified rib developed 
by the mouth. The respiratory circle of bones is the key to the 
skull. 
