590 Proceedings of the Royal Society of Edinburgh. [Sess. 
articulation distal to the malleus. I have failed to find this structure in 
any one of the four earlier embryos — the oldest of which shows the ossi- 
cular apparatus formed of true cartilage. Now if the mallear-incus articu- 
lation appears as the only mandibular articulation even at the time when 
the structures are formed of true cartilage, it follows that the later mandi- 
bular articulation must be both phylogenetically and ontogenetically 
acquired very much more recently. Therefore it seems clear that we must 
accept the reasoning of Gaupp and consider the quadrato-mandibular 
articulation as represented in the mammalia by the inco-mallear articula- 
tion. Further, the older view of Salensky, which Hertwig has followed, 
that both incus and malleus are split off, as it were, from the Meckelian bar, 
must be given up. The incus appears as a separate and distinct skeletal 
element developed from the proximal portion of the first visceral arch. 
The malleus appears at first, and continues to a very late period to be, a 
dependent portion of the first visceral bar. 
Before considering the relations of the stapes it appears advantageous 
to give a brief account of the courses of the main branches of those cranial 
nerves pertinent to our discussion, leaving, however, the elucidation of the 
facts rather largely to an examination of the figures. 
There are, of course, three nerves which bear an important relation to 
the auditory ossicles — the trigeminus, facial, and the vagus group, indi- 
cated in the figures by the numerals V., VII., and IX. The facial nerve 
exhibits the most important relations. The first branch (large superficial 
petrosal) is given off soon after the nerve leaves the ganglion. This 
branch runs medially and ventrally, and is of small moment here. The 
main trunk then bends slightly dorsal, along with the vena capitis 
lateralis, and runs almost directly caudal till it reaches the dorsal end of 
the hyoid bar. It is, moreover, dorsal to the tympanic cavity, so that all 
of the auditory ossicles must be ventral to it. At the proximal end of the 
hyoid it curves sharply latero-ventrally, sending off a small twig to the 
external ear and a larger branch — the chorda tympani. The chorda 
tympani skirts the hyoid bar and plunges into the mesenchymic mass 
surrounding the malleus and incus at the latero-caudal end, thence it runs 
cranio-medio-ventrally to emerge from the mesenchyme at the medio-caudal 
side of Meckel’s cartilage. Beyond the juncture of the main trunk and the 
chorda tympani the main nerve runs latero-ventrally and becomes un- 
important in the present discussion. Thus it will be seen that the auditory 
ossicles come to lie in a flexure of the seventh nerve — but that, nevertheless, 
they also lie in a position between the fifth and seventh nerves. 
It has been shown that the stapes arises medial and dorsal to the first 
