SKULL OF A HUMAN FETUS OF 40 MM. 391 



tales and ventrally by the dorsal extremities of the side-walls of 

 the etlimoidal region (fig. 11). It is thicker below than above, 

 its lower surface being keel-like, and passing horizontally forward 

 upon the lower edge of the nasal septum; the upper part becom- 

 ing increasingly thinner as we follow it ventrally, to coalesce 

 with the upper portion of the same structure. It represents a 

 transition from the flattened Balkenplatte to the thin nasal 

 septum, in which the median stem appears to undergo a torsion 

 of 90°, and forms the median delimitation of the narrow inner 

 recess of the orbit. 



The interorbital septum of man is homologous with the struc- 

 ture bearing the same name found in the Saurians, but, as is the 

 case in the other mammals, it is rudimentary when compared 

 with these lower forms. A theory which seeks to account for 

 the shortness of this septum when compared with the condition 

 in the lower forms is that, in the mammals, the nasal side-wall 

 has gradually grown backwards, to encroach more and more 

 upon the territory of the interorbital septum, to the advantage 

 of the nasal septmn (Mead). 



If we start at the floor of the sella turcica (fig. 1), and pro- 

 ceed upward and forward we come upon three successive steps, 

 at about equal intervals apart, formed, as we have seen, by the 

 lamina hypochiasmatica, the uppermost edge of the interorbital 

 septum, and finally, in the most cranial, which reaches the high- 

 est point of the chondrocranium, by the crista galli of the mes- 

 ethmoid (fig. 11). 



The orbital wings (figs. 1 and 10) are the cranio- ventral and 

 the larger of the paired lateral extensions of the sphenoidal 

 anlage. Each wing has the form of an imperfectly defined, 

 triangular plate, with the irregular base parallel with the median 

 plane, and the apex lateral and turned dorsally. The plate is 

 gently concave downward, forming the roof of the orbit; above 

 it takes part in the formation of the floor of the anterior cranial 

 fossa. Of the unions, two are with the central stem of the 

 chondrocranium, the dorsal and ventral roots, and one which 

 is elongated and broken by several foramina is with the side- 

 wall of the ethmoidal region. The two wings lie almost hori- 



