542 ON THE THEORY OF THE VERTEBRATE SKULL 



into the architecture of the skull, which enabled him to determine the 

 homologies of some of the most important bones of its upper arch 

 throughout the vertebral series. Rathke showed the singular nature 

 of the primordial cranial axis, and Reichert pointed out in what way 

 alone the character of its lower arches could be determined. For the 

 first time, the student of the morphology of the skull was provided 

 with a criterion of the truth or falsity of his speculations, and that 

 -criterion was shown to be Development. 



My present object is to lay before you a brief statement of some 

 of the most important results to which the following out of the lines 

 of inquiry opened up by these eminent men seems to lead. Much 

 of what I have to say is directed towards no other end than the 

 revival and justification of their views — a purpose the more worthy 

 and the more useful, since with one or two honourable exceptions — 

 I allude more particularly to the recent admirable essays of Prof 

 Goodsir — later writers on the Theory of the Skull have given a 

 retrograde impulse to inquiry, and have thrown obscurity and confusion 

 upon that which twenty years ago had been made plain and clear. 



I have said that the first question which offers itself is, whether 

 all vertebrate skulls are or are not, constructed upon a common plan, 

 and in entering upon this inquiry I shall assume (what will be 

 readily granted), that if it can be proved that the same chief parts, 

 arranged in the same way, are to be detected in the skulls of a 

 Sheep, a Bird, a Turtle, and a Carp, the problem will be solved 

 affirmatively, so far, at any rate, as the osseous cranium is concerned. 



Composition of the Skull of a Sheep (fig. i ). 



On examining a section of the cranium of a sheep, made either 

 along a vertical and longitudinal, or a transverse and horizontal 

 plane, a more or less completely ossified mass is observed in the 

 middle line below, which forms part of the floor of the cranial cavity, 

 but extends beyond it. This may be termed the ' craniofacial axis.' 

 Posteriorly it is a broad plate flattened from above downwards, and 

 is nearly parallel with the long axis of the cranial cavity ; but from a 

 point immediately behind the sella turcica, it becomes thicker and is 

 compressed from side to side, so that, at the anterior boundary 

 of the sella turcica, the craniofacial axis is much deeper than wide, 

 and assumes the form of a vertical plate. From the anterior boundary 

 of the cranial cavity onwards, or in its facial portion, the axial plate 

 is very deep and very thin, and a line drawn through its longitudinal 



