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Show 186 ON '.l'HE STRUC'rUUE OF THE KULL. th ther hand t l1 e h vo mandibular altogether b1 longs tod tLe on e 0 d I. scoral ar,c 'h w1 u . 1"e the hind r ern of th pa a. t o-qu.a .ra. te secon v c 'b t 1 8 belongs to the first, u . become c1 tach d from 1ts pnm1t1ve la .. t' 'th the ba lS crann. connec wn ;vi . 't l : o·inates as an o ification, which imme- The bai-n-occi pi a oub c.. • 1 ·d d d1. t 1 surroun d s an d 1·u c' los~ e the nd of the notoc 101 , an at een dYs I.n to t l1 e a aJ··1 C nt <a- rtilag . The e r-occipital • is de-exel o·p ed w.i th I' n t.h e ubstan e of the cartilagino'ud ~ crmln um l on eva ch si.d e of the b asi·- occi}>ital. .T he pa. rasp ·h no· i , 1o n t 10: oht 1er hand . d 1 d s a superfiCial os Ifi atwn In t le pon e on- JS eve ope a . f f d dn.u m' of t1 l e b ase f the skull and xt nd In ront o ' an 0 ' r 1h ']l belu.n d t l1 e p1' tu 1' ta r~ Y .1r'o s a in thi menlbrane. I e pre-maxi oo 1 ] and m'a xi'1 1 re have no cartilaainon pr d c sor ' nor Ltt ve t .te J b rrJ l t ' . 1 ~ . f the lower Jaw. . le pa a me IS dentary and angud a~ f ple~ in, the ant rior crus of the palatadeveloped aroun , I no Fig. 73. THE DEVELOPMENT OF 'l'HE SKULLS 0]' FISHES. 187 quadrate arch; the metapte1~ygoid in the same r~lation to its posterior crus; the quadrate bone, in its inferior process. The symplectic is a cortical ossification of the styliform part of the hyomandibular cartilage, the ossification of the rest of the latter giving rise to the hyomandibular bone itself (Fig. 72). In many osseous fishes, such as the Carp, the cartilaginous crani u1n disappears, with age, as completely as it does in Man ; but, in the Pike, it not only persists, but grows and enlarges with age, so that the relations of the cranial bones to cartilage, or to me1nbrane, can be investigated at any period of life. If the skull of an adult Pike be macerated, or, better, steeped for a short time in boiling water, a number of the cranial bones will separate with great ease from a sort of model of the skull chiefly composed of cartilage. This "cartilaginous skull " forms a complete roof over the cranial cavity (Fig. 73, A), whence it is continued, without interruption, to the anterior ond of the cranium, forming the narrow inter-orbital septum (I. Or.) and the broad internasal rostrum (Eth.), and· giving rise to two antorbital processes (Prj.), which separate the orbits from the nasal chambers, and are perforated by the olfactory nerve, and Ly the 11asal division of the fifth. The inter-orbital cartilage is interrupted by an oval space filled with membrane, just in front of the basi-sphenoid, so that it is continued to the lower end of that bone only by a slender cartilaginous rod, which passes into the stem of the Y-shaped basi-sphenoid (Fig. 7 3, 0). The cartilaginous basis of the skull, therefore, is not continued back along the floor of the canal for the orbital muscles. The roof of the orbital canal contains cartilage in the middle line, which is ahnost completely hidden in front by the extension towards one another of the horizontal laminm of the pro-otic bones. The under-surface of the inter-orbital septum and of the greater part of the cartilaginous rostrum is marked by a deep groove (a, Fig. 73, B), into which a median ridge of the parasphenoid is received. The bones which, being developed in perichondrium, are easily removed from the macerated skull, are the parietals, the fi·ontals, the bones 1.1. and 2.2. (Fig. 69), the squamosals (when |
