168 MAMMALIAN ANATOMY 



The internal or anterior surface of the occipital plate (Fig. Ill) 

 is smaller than the external surface, inasmuch as the upper part of the 

 external surface and the wide lateral borders are formed at its expense. 

 It has the form of an arch thrown over the foramen magnum. It is 

 narrower at the sides than above, and its lower margin is for most of 

 its middle distance the curved anterior superior margin of the canal 

 of the foramen magnum. At the sides it passes down into the 

 anterior surface of the paroccipital processes. The upper and outer 

 margin is the lateral border, articulating, as before mentioned, with 

 the interparietal, the parietal, and the mastoid of the temporal. 



The internal surface is concave from side to side and from above 

 downward, and faces forward. It is deeply marked by pits for con- 

 volutions of the part of the brain called the cerebellum. Of these 

 pits, a deeper median vertical pit, divided often into two by a trans- 

 verse ridge, covers the vermiform appendix of the cerebellum, but is 

 by no means constant or median and symmetrical. The groove for 

 the lateral sinus crosses the surface obliquely, beginning above on the 

 lateral margin at a point where the parietal and the temporal meet, and 

 ending in the jugular notch below. Under its inner margin, half- 

 way down, is the oval anterior opening of the posterior condyloid 

 canal. 



Inasmuch as the anterior and posterior margins of the foramen 

 magnum are separated by a considerable interval, namely, the thick- 

 ness of the occipital plate of the bone, there is formed between the 

 two an occipital canal, whereof the plane is directed from below 

 obliquely upward and backward. Its floor is slightly concave from 

 side to side and convex from behind forward, and in front is not 

 clearly defined from the floor of the basilar process. Each wall is 

 narrower above and below than half-way up, where it is enlarged by 

 the swelling produced on the posterior border by the prominent part 

 of the occipital condyle. The roof of the canal is narrow and concave 

 in both directions. On each side where the roof passes into the wall, 

 midway between the two borders, is the posterior opening of the 

 posterior condyloid canal, which pierces the bone obliquely and 

 appears again, as 'has been said, in the groove for the lateral sinus. It 

 transmits a vein. In front, behind a swelling at the point where the 

 wall passes into the floor, is the posterior opening of the anterior con- 

 dyloid canal, which opens anteriorly into the jugular notch. It trans- 



