DEVELOPMENT OE THE SKULL IN THE MAMMALIA. 
is the “articulare externum”—here the rudiment of the bony malleus (ml.). Opposite 
these parts the outer skin is somewhat enfolded ; this is the beginning, or fore margin, 
of the external auditory meatus (m.a.e.). The tympanic cavity ( c.ty.) broadens 
towards Meckel’s cartilage, and has a concave upper border; it is very large at this 
part, and has the epihyal ( e.liy.) below its lower, in turned recess. 
19 th Section (Plate 4, fig. 7).—This is from a short distance behind the last, and is 
through the posterior clinoicl wall, which is low and indistinct. The cochlea (chi.) is 
laid open at its anterior third, and the cartilaginous capsule is perforated above, and 
notched laterally, for the facial nerve (VII). Through the obliquity of the section the 
pituitary body ( py .) is still seen from behind; the internal carotid arteries (i.c.) are 
entering, right and left. 
The basis cranii is here at its narrowest part; this is the region of occipito-sphenoidal 
synchondrosis. The wall-cartilage is here of great depth; this may be called the 
supra-auditory tract of the chondrocranium (s.a.c.) between the hinder orbito- 
sphenoidal band and the completed occipital roof (Plate 5, fig. 1). The squamosal 
(sq.) and the parietal ( p.) are here seen cut through ; the latter also reappears opposite 
the involution of the skin forming’ the outer ear. Meckel’s cartilag'e is here seen 
where it is giving off the “ manubrium mallei,” and the bulbous form it has at this part 
gives the section the appearance of discontinuity between the head and the handle ; 
this is corrected by the figure of the dissection (Plate 2, fig. 4). 
This internal angular process of the lower element of the first arch is seen to push 
the lining of the drum cavity before it, in its growth downwards and inwards. Under 
and outside the cavity patches of cartilage are seen; these are parts of the annulated 
meatus, and the inner ring, on which the “os tympanicum ” or annulus is forming. 
Below the cavity, the epihyal is seen cut across ; below it the chorda tyrnpani 
branch of the facial nerve was seen in section, and from the manubrium mallei the 
tensor tympanic muscle ( t.trn .) is seen passing inwards and upwards. 
20 th Section (Plate 4, fig. 8).—The supra-auditory cartilage is not continuous, here, 
with the capsule; this tract is protected by the parietal (p.), and below the parietal 
a part of the squamosal (see fig. 7, sq.) is cut across. The large lateral lobes of the 
hemispheres are seen to cover the mid-brain ; below this the basilar artery (b.a.) is 
seen, and right and left of the basis cranii the internal carotids (i.c.) are entering. 
Here the basioccipital cartilage (b.o.) is like a thick warped plank whose convexity is 
below. The space between the capsule and the basal part, which was slight in the last 
section, is widened here, and will become considerably wider where the 9th and 10th 
nerves emerge. 
On this left side the cochlea (chi.) is cut through at its widest or proximal part, and 
the tegmen tyrnpani growing out from it covers the emerging facial nerve (VIP). 
Here the malleus (ml.) is sectioned so as to show a continuous growth of cartilage from 
the head down to the end of the manubrium, the inturned end of which lies on that 
part ol the infolded skin which becomes the membrana tyrnpani. Under the crescentic 
