lO 



NATURE 



{May 7, 1874 



When 'all the foregoing bones are stripped off, the 

 salmon's skull is far more comparable than in its perfect 

 state with that of an Elamobranch, being reduced to the 

 chondro-cranium, a cartilaginous structure, with certain 

 endogenous ossifications, but retaining to a remarkable 

 extent the characters of a "primordial skull." A side 

 view of the chondro-cranium is shown in Fig. 9 : viewed 

 from above it presents, like that of the ray, expanded 

 sense capsules, and a narrowed inter- orbital region ; the 

 walls of the brain-case are, however, much thicker, and 

 its cavity relatively smaller than in the preceding type 

 (see Fig. 8) ; the rostrum also is short, and the roof of the 

 skull or tegmen cranii produced into a strong ridge 

 (culmen cranii). The end of the snout divides into two 

 short processes (hypo-trabeculars, H.Tr), on each of 

 which two labial cartilages are borne (1^, 1-). 



The bones developed in the chondro-cranium of the 

 salmon very rarely come together so as to form sutures, 

 but are usually separated by considerable tracts of carti- 

 lage or synchondroses. Ankylosis only takes place in the 

 case of a single pair of bones — the orbito-sphenoids — 

 which are fused together in the mid-line, so as to form a 

 structure not unlike the "girdle-bone " of the frog. 



T.y.7. 



Fic 7 — SkuUof Silmon Pa pin tal Fr frontal Ni nasil S Etii 

 ■iupraell moidil S Or supra irbual Sb Or s b orb nl Lch lachry 

 mil Pmx pre ma\ lla Mx ma\illi Ju jiigal D ikntary A g 

 angular Op opercular S O^ sub operc ilar I Op i ter oper ular 

 P Op pre opercular Brs R branchio tegal rajs B Brs basi bran 

 chiosteeal S O supra o'-c pital Ep O epiotic Pt O pterot c PI 

 palatine Ms Pt meso plerjgoid Art articuhr Sy sjmplectic 

 O Hy glosso hjnl The cartiligmous parts arc dotted 



The hinder or occipital region of the skull is ossified 

 by four bones, which burround the foramen magnum, and 

 together form the "occipital segment;" these are the 

 basi-occipital (Figs. 8 and 9, B.O) below, the exoccipitals 

 (E.O) at the sides, and the supra-occipitals (S.O) above. 

 The first of these bears a concave surface or condyle 

 (O.C) for articuhtion with the first vertebra, the space 

 between the two being filled up with the remains of the 

 nolo, hord. The auditory capsules are strengthened by no 

 less than five bones : the prootic (Pr.O) iormed in the 

 anterior part of the capsule ; the opisthotic (Op.O) over 

 the ampulla, and the epiotic (Ep.O) over the arch of the 

 posterior semicircular canal; the pterotic (Pt.O) over 

 the arch and ampulla of the horizontal, and the 

 sphenotic (Sp.O) over the ampulla of the anterior canal. 

 The prootics of opposite sides meet in the mid-line 

 (Fig. 8), and form a bridge of bone on the base of the 

 skull, in front of the basi-occipital. Anterior to this 

 "prootic bridge," and completing the basis cranii, is a 

 small bone, Y shaped in section, the basi-sphenoid (B.S), 

 which, curiously enough, has no cartilaginous predecessor. 



but is ossified directly from membrane. Above this bone, 

 and in front of the sphenotic, the ali-sphenoids (As) are 

 found in the side-walls of the brain-case, and, together 

 with the basi-sphenoid below and the parietals above, form 

 the " parietal segment " of the skull. The " frontal 

 segment " has no basal element, the pre-sphenoid being 

 absent, but its side-pieces are represented by the co- 

 alesced orbito-sphenoids (O. S) The only remaining bone 

 in the skull proper is the large lateral ethmoid (L.Eth), 

 which occurs immediately behind the depression for the 

 nasal sac (Na). 



Certain very constant relations exist between these 

 bones and the cranial nerves. The trigeminal (V.), for 

 instance, always determines the prootic, as its third divi- 

 sion makes its e.xit just in front of that bone, or, in other 



V so 



^\rJHS 



110 



IH-O 



Fig. 8. — Longitudinal section of Salmon's Skull. Pa. S, para-sphenoid ; Vo. 

 vomer; K.O, basi-occipital; K.O, exoccipital ; Pr.O, prootic; B.S, 

 basi-sphenoid; As, ati-sphcnoid ; O..S. orbito-sphenoid ; O-C, occipitai 

 condyle; 1', 1^, labial cartilages; m.n.c, middle nasal cavity ; e.t.f, 

 ethmo-trabecular fissure. 



words, between the anterior boundary of the auditory 

 capsule and the parietal segment. The glosso-pharyngeal 

 and vagus (IX. and X.) in like manner limit the posterior 

 boundary of the ear capsule, passing out either between 

 it and the exoccipital, or through the front part of the 

 latter. The optic nerve (II.) passes between the parietal 

 and frontal segments, usually being bounded in front by 

 the orbito-sphenoid, and bshind by the orbito-sphenoid. 

 In the salmon a bar of bone grows across the trigeminal 

 notch of the prootic, so that part of the nerve passes 

 through a complete foramen. 



An interesting instance of the retention of embryonic 

 characters is seen in the slit marked e.t.f in the sectional 

 view. Fig. 8. This is a fissure in the otherwise solid 

 cartilage running forwards for a short distance from the 

 lower anterior angle of the orbito-sphenoid, and indicating 



Fw.9. 



WPr 



OS ]i „ „ 

 \ I ■!>» SO Sp^O 



_\ V / -'■ 



Fic. 9 -Skull of S..lmon, with investing bones and fa:'al orthes rem^vfd. 

 Op O, opistbmic ; Sp.O, sphenotic ; L.Eth, lateral ethmoid ; 11 Ir, 

 hypo-trabectllar 



the line of separation between the trabecular portion of 

 the skull and the part produced by the chondrification of 

 Its originally membranous walls ; this structure is called 

 the ethmo-trabecular fissure. In front of and above this 

 fissure is a large cavity (m.n.c) filled with fat, and open- 

 ing on the surface of the chondro-cranium beneath the 

 supra-ethmoidal bone ; there is no doubt that this seem- 

 ingly useless space represents the single nasal chamber 

 of the lamprey or hag. 



The structure of the facial arches, and the chief points 

 in the development of the salmon's skull, will be con- 

 sidered in the next paper. 



{'/o />( contiiuitd.) 



