DEVELOPMENT OF THE SKULL IN THE PIG. 



323 



above (a.sc, h.sc, p.sc.) and the cochlea below (cl.). The hollowed tegmen tympani {t.ti/.} 

 has in its hinder recess the head of the incus (/. ) ; the recess ends in a round cup-like 

 facet for the short crus of the incus, with its down-turned rounded head ; the " aceta- 

 bulum " for this head is finished, externally, by the squamosal, part of which, having 

 become adherent, is shown in the figure. Below this wall-chamber for the incus is the 

 fenestra ovalis with the enclosed stapes (f.ov., st.) ; the long axis of the oval space and 

 oval base of the stapes is upwards and forwards. The inturned hook of the long crus of 

 the incus is now coupled to the neat head of the stapes by means of an intermediate 

 bone, the " os orbiculare " (o.ob.), a special centre developed in ilie primary head of the 

 second postoral bar, which, limpet-like, applied itself to that periotic " bud " which 

 became the stapes, by a process similar to that which detaches the axillary buds in Lilium 

 tigrinim. In this figure the malleus is not given; it is shown in fig. 3 (ml.). The 

 processus gracilis (p.gr.) is reduced to a style, ending in fibrous tissue; the manubrium 

 (mb.) is flat and slightly arcuate; the "head," articulated with the incus, is elegantly 

 notched for this purpose, and fits on to the incus by a synovial joint, the miniature of 

 that by which the tibia fits on to the astragalus in this same animal. Between the 

 head and the manubrium the bone is thin, and is scooped externally ; the head sends 

 inward a rounded process (i.jp.m.), and the manubrium sends backwards an angular snag ; 

 this latter is for the attachment of the " tensor tympani" muscle. The little secondary 

 nucleus of cartilage which we saw developed between the dislocated incus and stylohyal 

 (Plate XXX. figs. 8 & 9, iJiy.) is now attached to the neck of the stapes by its broad 

 outer end, whilst its bluntly pointed distal end is buried in the fibres of the tendon of the 

 *' stapedius " muscle (sf.m.). This is the last efiect of the high degree of metamorphosis 

 exhibited by the second postoral bar of the Mammal. The fore edge of the exoccipital, 

 with its paroccipital spur (fig. 2, e.o.^iJ.oc.), is strongly clamped upon the auditory capsule ; 

 this is also made still stronger by the large posterior fiange of the overgrowing squamosal, 

 not shown in this figure. 



The under surface of the snout is also given at this stage, to show the complete 

 coalescence of the alae nasi with the recurved trabecular horns, and their continuation 

 backwards as the " recurrent laminae " {rc.c), also the alinasal external segment or 

 " appendix." The solid fore end of the snout, already full of small blood-vessels, is 

 ready to become the snout-bone for rooting ; this bone is formed in the ossified knees 

 of the trabeculse: the stunted, recurved prenasal cartilage is now undistinguishable 

 from the base of the septum nasi, formed by the complete coalescence of a large tract of 

 the trabecular bars, the long commissure of the foremost facial arch. 



Eighth Stage. — The Skull of a Pig 6 months old. 

 This makes a more convenient last stage than the adult, as here are still in existence 

 the greater number of the sutural landmarks, so soon to be largely obliterated, whilst 

 the change in general form is rather of interest to the zoologist than to the morpho- 

 logist. The long angular skull (Plate XXXVI. fig. 4, a,nd Plate XXXVII. figs. 1 & 2) 



2u2 • 



