DEVELOPMENT OF THE FACE. 17 



the condyle. In rodents the glenoid cavity is a narrow gutter in 

 which the plate-like condyloid process glides backwards and 

 forwards. The interarticular cartilage is developed in all the 

 Mammalia except the monotremes, and one or two marsupials 

 (Parsons). 1 It is probably a derivative of Meckel's cartilage (see 

 Fig. 10Z>). 



Development of the Tympanic Plate and Articular Emin- 

 ence. — If the chin be depressed the condyle of the jaw moves 

 on to the articular eminence (Fig. 13i?); if over- depressed 

 it springs over the eminence, and a dislocation is produced. 

 This is impossible in the early years of life, for at birth 

 there is no eminence and no glenoid cavity (see Fig. 14 A). 

 At birth the membrana tympani lies exposed on the surface 

 of the skull behind the condyle, supported in a fine osseous 

 hoop, the tympanic ring. The ring is imperfect above, and 

 there the flaccid part of the membrane occurs. By the second 

 year the ring has grown into a plate by sending out two 

 processes, which, as they grow out, unite and leave a gap between 

 (Fig. 14 B). This, as a rule, is soon filled up. By the 20th year 

 the tympanic plate is three-quarters of an inch long, forming the 

 bony floor of the external meatus and the posterior wall of the 

 glenoid fossa, which in man is remarkably deep. It protects 

 the meatus from the condyle; every year until the 20th the 

 bony meatus gets longer, while the fibro-cartilaginous part 

 becomes relatively shorter. In the adult the bony part forms 

 two-thirds of the meatus. As the tympanic plate grows outwards, 

 the membrana becomes less easily accessible to the surgeon 

 (Fig. 14 G). The plate also grows inwards to form the floor of 

 the bony part of the Eustachian tube and downwards to form 

 the vaginal process, to which the upper end of the carotid sheath 

 is attached (Fig. 40, p. 54). Gadow regards the tympanic plate 

 as the representative of the quadrate bone of birds and reptiles. 



THE STOMODAEUM. 



The stomodaeum or primitive buccal cavity is the depression 

 or narrow pocket formed between the fore-brain above and 

 the mandibular arch below. It is bounded laterally by the 



1 " Joints of Mammals," Journ. of Anat. and Physio., Vol. XXXIV. 



B 



