166 ON THE STRUCTURE OF THE SKULL. 



auditory apparatuses are fixed within their chambers, while the 

 eye is freely moveable within the orbit. 



Thus, for the Pike, I may repeat the phraseology which I 

 employed in giving a general description of the skull of Man. 

 It consists of an axis, of upper and lower arches, and of chambers 

 for the sensorv organs. 



The next point is to ascertain how far this correspondence, 

 thus traced generally, extends into the details of the composition 

 of the skull ; and here we may conveniently begin, as before, 

 with the study of the cranio-facial axis. 



Viewed as a whole, this axis is rounded and thick behind, 

 com pressed from side to side in the median region, and thickened 

 and depressed in front. It is composed, as I have said, partly 

 of bone and partly of cartilage. Behind, it consists of a single 

 well-ossified mass (B.O.), which offers, posteriorly, a deeply ex- 

 cavated conical articular facet, quite similar to that presented 

 by the body of the first vertebra, with which it articulates. 

 Anteriorly, it is also excavated in the middle, its conical cavity 

 terminating the canal for the orbital muscles behind. Its upper 

 face forms the hinder part of the floor of the cranial cavity and 

 the inferior boundary of the occipital foramen. Its lower face is 

 bevelled off in front, and articulates with the hinder part of the 

 upper face of the bone x, Fig. 68. 



Laterally and posteriorly, it articulates with the bones (E. 0.), 

 which constitute the lateral boundaries of the occipital foramen ; 

 while, laterally and anteriorly, its deeply-excavated surface is 

 free, and forms part of the deep chamber in which the sacculus 

 of the auditory organ is lodged. The greater part of this bone 

 is solidly ossified throughout, but its conical anterior cavity is 

 lined by a thin shell of bone, which is separated by a continuous 

 layer of cartilage, thicker above than below, from the rest of the 

 osseous mass. 



In a longitudinal section (Fig. 68) of a fresh Pike's skull, the 

 upper part of this layer of cartilage is readily seen, and can be 

 traced without interruption, from the axis of the bone under 

 description as far forwards as the posterior margin of the pitui- 

 tary fossa, and therefore, for a long distance in front of the 

 anterior termination of the bone B.O. The layer of cartilage 



