Lect. III.] THE STIRRUP AND COLUMELLA. 79 



upper end, to partial starvation, in order that it may he 

 small enough to form the special malleus. 



But the wrapping and binding of the cartilaginous 

 lower jaw in bony splints — three on the fore part, and 

 three on the hinder, proximal, or upper part — is as old as 

 the Ganoid Fishes of the Old Red Sandstone, who formed 

 their mandibles in that way ; and the living Sauropsida 

 — all known Birds and Eeptiles — to this day form their 

 mandibles in the same manner ; it is a new thing when 

 the hinge-part of the lower jaw is starved, so that it 

 may shrink into the little auditory malleus. 



Character 7. — This also is a Mammalian modification 

 — a new specialisation of an old structure — when the 

 innermost bone of the ear-chain grows so as to resemble 

 a stirrup. It does so because of a peculiar l)ranch of the 

 " common carotid artery," which passes in the early 

 embryo close to the opening (fenestra ovalis) of the vesti- 

 bule, to press its way to, and unite with, the " inferior 

 maxillary artery." In its passage it trespasses on the 

 ground belonging to the topmost segment of the arch 

 of the tongue (pharyngo-hyal), and a compromise is 

 made by the cartilage hardening round the little artery. 

 But in several of the Marsupials this does not take 

 place; nor in several of the lower Eutheria, e.g., some 

 of the Edentata. The Monotremes, also, the Saurop- 

 sida, and the Amphi1)ia, have this element imperforate, 

 and thus it has received the name of the " auditory 

 columella." 



