90 



LECTURE V. 



Disarticulated epenccphahc 



arch, viewed from behind : 



Gadus Murrhua. 



The first and lowest, called basi-occipital [ib. l) *, is a short, 

 strong, sub-rhomboidal bone, sub-cylindi-ical and truncate posteriorly, 

 where it is excavated to form the articular 

 cavity, united by a jelly-filled capsule with 

 the corresponding hollow cone on the fore- 

 part of the body of the atlas; the anterior 

 pointed end of the basi-occipital is wedged 

 into the basi-sphenoid, fitting and filling up 

 the deep posterior cleft of that bone. The 

 basi-occipital supports the ' oblong prolonga- 

 tion,' of the spinal chord in the skull ; and 

 on each side offers a rough articular surface 

 for sutural union with two lateral bones, the 

 ex-occipitals {ib. 2, 2) ; behind which the 

 basi-occipital, also, sometimes receives the 

 anteriorly projecting base of the neural arch of the atlas, which, 

 in the Cod, is wedged into the posterior angle between the basi- 

 and ex-occipitals, and is firmly united to them by broad sutural 

 surfaces. 



The articular cup for the atlas varies from the deep conical excavation 

 seen in the Carp, to the almost flat surface in the Holibut ; it is ex- 

 tremely rare to find, as in the Fistularia, the basi-occipital presenting 

 a convex surface for articulation with the body of the atlas. In many 

 fishes the under part of the basi-occipital is expanded and excavated ; 

 in the Carp the under surface of this part forms a broad triangular 

 plate, which supports the large upper pharyngeal grinding tooth | ; 

 in the bony Gar-fish {Lepidosteus) the basi-occipital developes two 

 plates from its upper and outer angles, which complete the foramen 

 magnum and support the ex-occipitals above. 



The ex-occipitals (neurapophyses of the occipital vertebra, ib. 2, 2) 

 present, in the Cod, the form of oblong, sub-quadrate bones, thick, 

 and with two rough deeply indented articular surfaces below, but 

 expanded and produced outwards above ; they encircle the epen- 

 cephalon, over-arching and often meeting above it, and completing 

 the contour of the foramen magnum. They are perforated for 

 the passage of the nervi vagi, sometimes for the first spinal or 

 hypoglossal nerve ; the foramina being unusually large in the Carp- 



* The synonymes and corresponding numbers and letters of the bones of the skull 

 are given in the tabular conspectus appended to this Lecture ; the numbers of the 

 bones in the text correspond in each of the figures. 



f Dentigerous processes are developed from the under part of the cervical verte- 

 bral centres in the Coluber scaler of Linna>us. 



