74 LECTURE IV. 



terior to the median ridge there is a small fossa, (in the young 

 Squatina a foramen,) the last trace of the pituitary canal : the basal 

 cartilage then expands to form the lower border of the groove which 

 receives the palatine process or point of suspension of the palato- 

 maxillary arch, and the cartilage then suddenly contracts, and is con- 

 tinued forwards to form the vomerine antei'ior base of the cranial 

 cavity. The fibro-membi'anous parietes of this cavity are every 

 where covered with, or converted into, the same firm granular carti- 

 lage as the base, save at the anterior and upper end, where a large 

 fontanelle, closed by the primitive membrane, remains : the cartila- 

 ginous walls ai'e perforated by the exit of the cerebral nerves, and the 

 spinal chord. 



The cranial cavity is not moulded upon the brain, but is of larger 

 size ; it communicates merely by the nervous and vascular foramina 

 with the acoustic labyrinth, which is buried in the thick lateral cranial 

 cartilage. This insulation of the ear- capsules from the brain-case is a 

 high grade of development common to all the typical Plagiostomes : in 

 the Chimajrte the separation is only partial. The large pituitary de- 

 pression, or ' sella,' marked by a ridge across the floor of the cavity, 

 indicates the compartment between the orbits for the mesencephalon, 

 and in front of this is the wide prosencephalic, or cerebral, compart- 

 ment, which communicates by the two lai'ge foramina with the nasal 

 cavities, and, in the diy skull, opens forwards by the wide persistent 

 fontanelle. In the vertical lateral cartilaginous walls of the cranium 

 we recognise the part representing the great ala of the sphenoid by 

 the two perforations answering to the foramina oralia and rotunda for 

 the exit of branches of the fifth pair of nerves. The orbital alte of 

 the sphenoid are indicated by the foramina optica ; the part cor- 

 responding to the bones in osseous fishes, called by Cuvier " frontaux 

 anterieures," by the olfactory foramina, and by their articulation with 

 the palatine process of the maxillary arch. Two broad and thin car- 

 tilaginous plates, from the upper and anterior walls of the cranium, 

 support anteriorly the nasal sacs, and thence extend backwards and 

 outwards over the sides of the anterior half of the cranium forming 

 the roof of the orbits. Two longitudinal and vertical ridges, which 

 rise from the posterior and lateral cranial parietes, extend outwards 

 at both extremities in the form of strong triedral conical transverse 

 processes. The anterior one forms the post-orbital process ; the pos- 

 terior answers to the mastoid ; between these is a long cavity lodging 

 the temporal muscle, and beneath this the parallel articular cavity 

 for the tympanic pedicle. The extreme point of the mastoid is per- 

 forated by a mucous canal extending to the upper surface of the 

 back part of the skull. The post-orbital processes touch, and some- 



