136 ELEMENTARY ANATOMY. [LESS. 



In this way the peculiarly expanded condition of the 

 cerebral chamber of man may be appreciated. Taking the 

 basi-cranial axis as a fixed line for comparison with other 

 animals, we see that in him (Fig. 91) the cerebral chamber 

 is of great vertical extent, at the same time that its anterior 

 portion is so expanded as to open out the olfactory angle 

 from 90 to much beyond 180, and to similarly open out the 

 occipital angle circumstances in which man widely differs 

 from even the highest members of his class and even of his 

 order. 



In the fact that the inside of the arch of the skull is marked 

 by depressions corresponding with the cerebral convolutions, 1 

 man agrees with those members of his class which possess 

 such convolutions. For in that class the cranium closely 

 invests the brain, so that a cast of its cavity well exhibits the 

 general features of the cerebral surface. This character, 

 however, is by no means universal in the Vertebrates, for the 

 cerebral cavity does not contract with the lessening pro- 

 portions of the cranial nervous centres, there being (e.g. in 

 Fishes) a large quantity of soft fatty substance interposed 

 between those centres and the cranial walls. 



The cranial cavity in man overhangs the orbits, but does 

 not descend between them. This is a condition which 

 obtains very often, but by no means universally ; for that 

 cavity may be not only prolonged between the orbits, but 

 considerably beyond them. We find this in Serpents, in 

 Batrachians, and in many Fishes (e.g. the Carp family), while 

 in Birds, Lizards, Crocodiles, and Chelonians the cranial 

 cavity suddenly contracts, and there is an interorbital septum 

 only. 



The middle of the dome of the skull may be produced in- 

 wards as a median, longitudinal, bony plate by ossification of 

 the falx. This is the case e.g. in the Ornithorhynchus and 

 in the Sea Lion. 



An ossified tentorium may exist, as in some Spider 

 Monkeys, in the Racoon, and others. 



The base of the interior of the skull may present those 

 differences already noticed in describing the several cranial 

 bones. In man's class it is divisible into the same three 

 fossae as in him. This division, however, is not similarly 

 marked in lower forms. 



(i) The anterior fossa is relatively much less extensive in 

 man than it is in most animals. Even in the Apes the 



1 For these see Lesson VIII. 



