22 THE A^OYAGE OF H.M.S. CHALLENGEE. 



Seventh Section. — Here (PL V. fig. 1) the narrow fore part of the hemispheres (C la) 

 are severed, and the thick orbito-sphenoids {o.s.) on which they lie are very nearly 

 straight in section. The nasal passage is not so high, and is bent inwards at the middle; 

 the floor {n.p.) turns upwards, thickening there. The perpendicular ethmoid (i.tr.) is 

 much lower, and is not continuous, yet, with the alte {o.s.), and it is to be observed that 

 this partition is still a single cartilage. 



Eighth Section. — We have now reached the fore edge of the crystalline lens which 

 is shown in situ in the eyeball (PI. V. fig. 2, e.) ; and the naso-palatine canal {i.n.) is a 

 small passage with its convexity inwards ; granular tracts on each side show where the 

 maxillaries and palatines (inx.,pa.) will be. 



The internal rectus muscle is cut across, and part of the inferior rectus is shown 

 lengthwise {in.r.,if.r.) ; and here the hemispheres (C la) are enlarging, and lie in 

 the orbito-sphenoidal trough (o.s.). These plates, which in the former sections were 

 free below, are now continued downwards on each side of the orbital septum, which is 

 seen to be composed of a pair of flat jolates, embracing a middle piece, which is round 

 below, and sharp above. The thin plates are the trabecules [tr) which end behind the 

 front, when the last section was made ; the}^ send upwards the orbito-s]Dhenoids as in 

 the Batrachia, and indeed, in all other types, except when these are independently 

 developed, as in the Ophidia. The middle plate, or " intertrabecula " {i.tr.), forms all 

 the septum nasi and perpendicular ethmoid at this stage ; here it is the " presphenoid," 

 for it finishes the " anterior sphenoid " below. 



Ninth Section. — This section (PL V. fig. 3) is from a little behind the last, and 

 shows, on the whole, the same structures ; but the hemispheres (C la) are much larger 

 here, and the naso-palatine passages are now open to the roof of the mouth {n.g.) ; they 

 have formed the internal nostrils {i.n.). 



Tenth Section. — This section (PL V. fig. 4) is behind the inner nostrils, and behind 

 the stem of the orbito-sphenoids (o.s.), the part which is continuous with their root, the 

 trabeculse ; here, in front of the common optic foramen, the presphenoid is composed 

 of a low intertrabecular wedge, and the two flat trabeculae {i.tr.,tr.) ; they arise above the 

 middle plate, and it descends below them, forming a rounded keel to the interorbital 

 septum. Part, both of the superior and internal rectus muscles {s.r.,in.r), are cut 

 through, between the large eyeball (e.), and the widest j)art of the orbito-sphenoid {o.s.), 

 which, of course, is widest where it holds the most bulbous part of the hemisphere (C la).* 



Eleventh Section. — Here (fig. 5) the " thalamencephalon " (C 1) is seen below the 

 wide hemisphere (C la), and the trabeculae {tr.), at this part, are thick above and below, 

 and thinned out in the middle ; the intertrabecula {i.tr.) is now oval in section. 



Twelfth Section. — We now come to the ojatic nerves, and their chiasma (fig. 6, 2), 

 and their entrance into the eyeball to form the retina {rt.). The thalamencephalon is 

 surmounted by the hemispheres (C 1, C la), and between these above, part of the pMiea/ 



