THE SK ULLS OF !\I A.MMALIA. 257 



A line drawn from the hinder extremity of the basi-occipita] 

 to the uppermost part of the junction between the presphenoid 

 and the ethmoid, may be called the line of the axis of the basis 

 cranii, or the " basi-cranial liner 



A second line, drawn from the premaxilla to the basis cranii 

 through the junction of the vomer with the ethmoid, traverses 

 the axis of the facial part of the skull, and may be termed the 

 line of the axis of the basis faciei, or " basi-facial liner This 

 line, if produced upwards and backwards, will cut the foregoing 

 so as to form an angle open downwards, which I shall term the 

 " craniofacial angle." 



A third line, drawn from the end of the basi-occipital bone 

 to the posterior edge of the supra-occipital in the median line, 

 will give the general direction of the plane of the occipital 

 foramen, or the occipital plane. The angle it forms with the 

 basi-cranial line is the " occipital angle" 



A fourth line, drawn from the torcular Herophili, or junction 

 of the lateral and longitudinal sinuses, through the middle of a 

 plane joining the tentorial edges of the pro-otic bones, will give 

 the general direction of the tentorium, or, in other words, of the 

 demarcation between cerebrum and cerebellum.* This line, 

 therefore, may be taken to indicate the " tentorial plane." The 

 angle it forms with the basi-cranial line is the " tentorial angle." 



A fifth line, drawn through the median junctions of the crib- 

 riform plate of the ethmoid, with the frontal above and ante- 

 riorly, and with the presphenoid below and posteriorly, will give, 

 in the same general way, the u olfactory plane." The angle it 

 forms with the basi-cranial line is the " olfactory angle." 



Lastly, the longest an tero -posterior measurement of the 

 cavity which lodges the cerebrum will give the " cerebral length." 



Having defined these lines and planes, the following general 

 rules may be laid clown : — 



1. The lower Mammalia have the basi-cranial line longer in 

 proportion to the cerebral length than the higher. Taking the 

 length of the basi-cranial line as 100, 1 have observed the cerebral 



* Of course no straight line can give this boundary with exactness, as the co- 

 adapted surfaces of the cerebrum and cerebellum, and consequently of the interposed 

 tentorium, are curved in all directions. 



S 



