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62 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



therium by the great number and capacious size of the air-cells which are in 

 communication with it : these extend over all the upper, lateral, and back parts 

 of the cranial cavity, as far even as the upper boundary of the foramen magnum : 

 they also occupy the anterior two-thirds of the basis cranii. The external con- 

 figuration of the skull would, therefore, afford a very inadequate or rather 

 deceptive notion of the capacity of the cerebral cavity, were not the existence 

 and magnitude of these sinuses known. The interspace of the outer and inner 

 tables of the cranium are separated above the origins of the olfactory ganglia 

 for the extent of three inches : above the middle of the cerebrum they are an 

 inch and a half apart ; at the sides of the cranium the interposed air-cells are 

 from one to two inches across; at the back part of the cranium about one inch. 

 The sinuses have generally a rounded form. 



The foramen rotundum, (through which in figure 3 a probe is represented 

 as passing), and the foramen ovale are situated close together, within a common 

 transversely oblong depression (i). The carotid canal (g) opens into the outer 

 side of the commencement of this wide channel, which conducts the great fifth 

 pair of nerves to the outlets of its two chief divisions. 



The petrous bone projects into the cranial cavity, in the form of an angular 

 process with three facets : the foramen auditorium internum (A), and the aque- 

 ductus vestibuli, are situated on the posterior facet. Immediately behind the os 

 petrosum is the foramen lacerum jugulare (/), situated at the point of convergence 

 of the vertical groove of the lateral sinus, with a groove of similar size continued 

 forwards from above the anterior condyloid canal. The plane of the internal 

 opening of this canal (c, fig. 3) is directed obliquely inwards and backwards, and 

 the lateral wall of the foramen magnum behind the foramen condyloideum slopes 

 outwards to the edge of the condyle. Immediately internal to the foramen con- 

 dyloideum is a small vascular foramen conducting a branch of the basilar artery 

 into the condyloid canal, for the nourishment, doubtless, of the great lingual 



nerve. 



In the relations of the plane of the internal orifice of the anterior condyloid 

 foramen with that of the foramen magnum, we search in vain for a corresponding 

 structure in any of the Mammiferous orders, save the Edentata:* and among 

 these the Orycterope comes nearest the Glossothere in this respect. In the 

 degree of development of the internal osseous ridge giving attachment to the 

 tentorium cerebelli, the Ant-eaters and Armadillos more resemble the Glossothere 

 than does the Orycterope ; in which a continuous bony plate arches across the 

 cranial cavity : in the Manis a still greater proportion of the tentorium is ossified, 



* In the monotrematous Echidna, the large canal for the lingual nerve has a widely different direction and 

 course from that in the placental Edentata. 



