OCR Text |
Show lSS ON THE S TRUCTURE OF THE SKULL. these are not anchylose d wi th the opistb tic bon s),* the vomer, and the parasphen~id. 'fications of the substance of the b hiCh as OSSI .1 • The ones w. '. lf ·e not thu separable, are the basi-, 'l . . s craniuJn Itse ' ai . . 1 1' carti agmou . . 1 tl e three penotic bones, t 1e a 1- d supra-ocCipita s, 1 f ] espxh-,e naonid s, the basi.- sp1 l enm· a ' the post-frontal ' the pre- ronta s, b 3 3 (Fio·s G9 and 73 ). the rrho nes . .. a certb· n: n nse, tll e adult skull of the Piko may Le said tou sr, Ipnr esent., <.' •• tent form a condition of tho skull Ill a per IS ' I . l . t ··tnsitory in l\Ian. . w ucL 1 tJ St hI ( 'des oftl 1 e I1 uman ~'mtal carti1aginou crannlm grow lU.J ' • e e. Sl . l roof of the skull; let the pre-sphenmclal, up an~ umte I~ t 1: ortion be greatly longatod; let no ethmmdal, ~nd ~ntern~sal r . the pr - phenoidal and orbitodistinct ossrficatwn ta e p actl In ·t an werino· to the lamina . d 1 . 0'. s or Ill le par c. • sphenm. a ~~?lOll '. e the ba i- phenoicla1 o i.fi ation remmns perpendwuhuis, whil . , ld I)Ut on th most important yery sma1 1 , and that cranium '"on 'k and striking characters of that of the PI . . -1-ed in the lrulls of Fishes if'l really a * How far the bone which I bavc m~ ~o q~:::uno ·al of R ptilcs, Bird' anu membrane bono an~l th !10mologu t: rouo-h ;c-in vestigntion. 1\Ir. Pnrker is of Mammal ' is A. qnc twn wln ~ n eds <l th bomolo!!uc not of the squamosal, opinion that it is really n. C~l:tila? bonl . ~n 1 finds wollo de~cloped in the periotic b t f an indep •ndent o ·Ificnbon, w uc 1 10 . , c.ua psoul e<o f tllo Mole and 'hr w, and terms tho "It rotw.' 18fl LEC'rUT~,E X. ON ~rfiE STRUCTURE OF THE SI{ULL. rrHE SKULLS OF ] ISHES. IT has been seen that the skull of Man and that of th Pn\:e agree in passing, in a similar order, through similar developmental stages. Each, nt first, is a membranous cranium, its walls being composed of indifferent tissue, with the exception ,. of that small part of its base which is occupied by the notochord. The greater part of the substance of each becomes chondrified, and thus that cartilaginous cranium is produced, which is a temporary structure in the lVIan, but a persistent one in the Fish. Neither in the membranous, nor in the cartilaginous state, does the cranium of either Man or Fish present any traco of that segmentation which becomes obvious in the third condition, when, by the deposit of calcareous salts aronnd certain centres, either in the cartilaginous cranium or the adjacent membrane, the bony cranium is developed. These three conditions of the skull are rnanifested, in the same order of succession, by all vertebrate skulls which becotne completely ossified ; but the crania of many vortobrated animals remain throughout life in the second state, or in a condition intermediate between that and the third, while the skull of one of the Vertebrata persists in a state which can only be regarded as a modification of the membranous craniun1. Hence I shall proceed to describe the leading modifications of the Vertebrate Skull under these heads :-A. The membranous cranium. n. Tho cartilaginous cranium. 0. The cartilaginous craniu,m, with |