THE SKULL. 



485 



py 



tkh 



The incus is articulated with the quadrate end of the mandibular 

 arch, and its rounded head 

 comes in contact with the 

 stapes (fig. .342, st) which 

 is segmented from the fe- 

 nestra ovalis. 



The main arch of the 

 hyoid becomes divided into 

 a hypohyal [h.h) behjw and 

 a stylohyal {st h) above, and 

 also becomes articulated 

 with the basal element of 

 the arch behind (hh). 



In the course of further 

 development the Meckel- 

 ian part of the mandibular 

 arch becomes enveloped in 

 a superficial ossification 

 forming the dentary. Its 

 upper end, adjoining the 

 quadrate region, becomes 

 calcified and then absorbed, 

 and its lower, with the ex- 

 ception of the extreme 

 point, is ossified and sub- 

 sequently incorporated in 

 the dentary. 



The quadrate region remains relatively stationary in growth as 

 compared with the adjacent parts of the skull, and finally ossifies to 

 form the malleus bone of the ear. The processus gracilis of the 

 malleus is the primitive 

 continuation into Meckel's 

 cartilage. 



The malleus and incus 

 are at first embedded in 

 the connective tissue ad- 

 joiningthe tympanic cavity 

 (hyomandibular cleft, vide 

 p. 435) ; and externally to 

 them a bone known as the 

 tympanic bone becomes de- 

 veloped so that they be- 

 come placed between the 

 tympanic bone and the 

 periotic capsule. In late 

 foetal life they become 

 transported completely 

 within the tympanic cav- 



pa.ch. 



Fig. Sil. Embkyo Pig, two-thieds of an inch 



LONG ; ELEMENTS OF THE SKULL SEEN SOMEWHAT 

 DIAGRAMMATICALLY FROM BELOW. (FrOm Paiker. ) 



pa.ch. parachordal cartilage ; nc. notocbord ; 

 au. auditory capsule ; py. pituitary body ; tr. tra- 

 beculfB ; c.tr. trabecular cornu ; j)n. prenasal car- 

 tilage ; e.n. external nasal opening ; ol. nasal cap- 

 sule ; p.pg. palatopterygoid tract enclosed in the 

 maxillopalatine process; mn. mandibular arch; 

 hy. hyoid arch; th.h. first branchial arch; la. 

 facial nerve; 8a. glossopharyngeal; 86. vagus; 

 9. hypoglossal. 



FiG. 342. Embryo Pig, an inch and a third 



LONG ; SIDE VIEW OF MANDIBULAR AND HYOID ARCHES. 

 The MAIN HYOID ARCH IS SEEN AS DISPLACED BACK- 

 WARDS AFTER SEGMENT.\TI0N FROM THE INCUS. (FrOm 



Parker.) 



tg. tongue ; mh. Meckelian cartilage ; ml. body 

 of malleus ; mh. manubrium or handle of the mal- 

 leus ; t.ty. tegmen tympani ; i. incus ; st. stapes ; 

 i.hy. interhyal ligament ; nt.h. stylohyal cartilage ; 

 h.h. hypohyal ; b.h. basibranchial ; th.h. rudiment 

 of first branchial arch ; la. facial nerve. 



