40 THE VOYAGE OF H.M.S. CHALLENGER. 



The endocranium (seen in PL XII. figs. 1-3, cleared of the investing bones, and 

 palatine membrane bones) is a very remarkable structure, extremely large behind, 

 reduced to a vertical plate .with alae in the middle, and enlarging into a piUow-shaped, 

 double pouch in front. This form would not be so remarkable if the eyeballs were 

 retained in the figures, with the capsules of the ears and nose ; they are indicated in the 

 sectional views. 



There is a large synchondrosial tract, to be lessened afterwards, between the basi- 

 occipital and basisphenoidal regions {b.o.,b.s.), and the lateral parts of the latter are still 

 unossified ; in the adult there is only this one bone in the posterior sphenoidal sclero- 

 tome, and none in the anterior. 



The basisphenoid is formed as an ossification of the end of the notochordal sheath, 

 of the contiguous parts of the investing mass, running upwards as the j)ost-clinoid wall, 

 and of the newly-coalesced ends of the trabeculse and intertrabecula, up the bottom 

 of the deep notch behind the common optic foramen (PI. XIL figs. 1, 2, 3, h.s.). The 

 internal carotid arteries find their way into the " sella " between these outer and 

 middle bars, and the bony substance growing well around them, makes the bone strong 

 beneath the pituitary body, for there is no open space left there ; the cujaped " sella 

 turcica" (PI. XII. fig. 3, j^y.) is only imperfect below where the arteries enter. 



Below (fig. 2, b.s.), the bone is thick and trilobate, growing backwards towards the 

 basioccipital, and outwards towards the quadrate and tympanic cavity ; above (fig. 3), 

 the squarish bony mass is notched, right and left, to form the large foramen ovale (5). 



Between these two endocranial regions — the occipital and posterior sphenoidal — 

 the large ovoidal auditory organs {au.) are imbedded. Seen from the inside (PI. XI. 

 fig. 2), they present the appearance of a trimerous, syncarpous fruit, the three bony 

 centres — prootic, epiotic, and opisthotic {pro. ,€2^., op.) giving them this likeness ; between 

 these there is a wide triradiate synchondrosis. 



The sigmoid crest or selvedge running dowii in front is the remains of the ali- 

 sphenoid [al.s.), a deep chink behind separates the mass from the occipital wall (e.o.) ; 

 a shallower groove divides the capsule from the basal plate, whose sphenio-occipital 

 synchondrosis runs from side to side uniting the right and left stems of the triradiate 

 periotic cartUage. 



On account of the large size of the occipital roof, and .its far growth forward, the 

 epiotic {ep.) forms the upper piece, and the pro otic [loro), which is almost twice as 

 large, lies below as well as in front ; this bone is perforated by the branches of the last 

 pre-auditory nerve, the facial (7), with its specialised dorsal branch, the auditory (8), 

 also in front, the prootic is notched by the trigeminal (5). 



The opisthotic (0^9.) is intermediate in size between the other two ; it is an exactly 

 posterior shell, triangular within, but, behind, growing out into the paraoccipital wing, 

 where it always appears as a large distinct bone when the skull is looked at externally. 



