DEVELOPMENT OE THE SKULL OE THE COMMON EEOG. 
169 
The pterygoid bar {jpg.) is very uniform in size, but becomes broader as it passes into 
the suspensorium ; the orbitar process of the metapterygoid (Plate VIII. figs. 3, 4, or.jh) 
is now overlapped by the temporal or squamosal (sq.) ; it is a mere semielliptical expan- 
sion, convex externally and concave within. 
Meckel’s cartilage {mk.) has greatly increased in length, like the palato-pterygoid 
bar. The quadrato-symplectic lialf-cleft ( : qu.sy ) is still quite visible, the infrahyoman- 
dibular ( i.hm .) is turning its inner convex face forwards, which it will do more and more. 
Between its upper margin and that of the sessile “ orbitar process” is seen the narrow 
metapterygoid connective (Plate VIII. figs. 3, 4, m.jpg .), the upper part of which has now 
coalesced with the prootic region of the ear-capsule. Mesiad of the infrahyomandibular 
(Plate VIII. figs. 4, 7"), the “ portio-dura” nerve escaping from the posterior angle of the 
foramen ovale” passes in front of the periotic cartilage; it then runs downwards and 
forwards (Plate VII. fig 5, 7°) on the inside of the posterior margin of the suspensorium, 
and halfway down divides into a large branch, the “ chorda tympani,” for the mandible, 
and two lesser hyoid branches”*. 
Por the sake of clearness of ideas we may now recapitulate the changes undergone by 
the first and second postoral bars. The mandibular pier is now scarcely two-thirds the 
length of the free mandibular ray or articulo-meckelian bar (Plate VII. fig. 5, ar., mk ) ; 
and the angle formed by the upper or suspensorial part with the basicranial axis is be- 
coming almost a right angle : thus the distance between the two points of the trabecular 
and mandibular bars, which was at first so small as to be joined by a very narrow con- 
nective, are now united by the long pterygo-palatine rod. That part of the hyoid pier 
which first coalesced with the mandibular is still one with it ; but the free upper head 
has now become segmented off as completely as the recurved lower half of the arch, 
namely the “ stylo-cerato-hyal.” There is therefore now a free “ suprahyomancli- 
bular,” an “infrahyomandibular” confluent with the mandibular pier, and sending 
down behind that bar a secondary symplectic selvedge. Finally, there is the proper 
hyoid cornu comprising the styloid and cerato-hyal regions, the latter coalesced now 
with its basal element. 
The hyoid now becomes a long narrow bar, very unlike its earlier conditions, and 
becoming not only loosely attached to its original fossa, but also gaining continually a 
more basicranial position (Plate VII. fig. 5, hy., st.h.). Towards its base the bar thickens 
and projects forwards as a hypohyal process ( hy . li) before it is bent backwards to unite 
with the still soft basal element; the latter part is confluent with the basibranchial 
{ b.br .). The upper part of the hyoid pier is that in which the metamorpliic changes are 
most remarkable and of greatest interest. 
The more highly magnified views (Plate VIII. figs. 3, 4) give the best idea of these 
parts. The “ suprahyomandibular,” losing all relation to the hyoid arch, becomes now 
part of the middle ear-, and a cartilaginous “opercular” of a crescentic shape (indicated 
by a dotted outline in Plate VIII. fig. 3, a.t .) forms the outer part of the auditory appa- 
* I ’nave carefully studied tlie distribution of the “ portio dura” in the Bull-frog. 
MDCCCLXXI. 2 A 
