Chap. VIII.] THE SKELETON. 161 



grow up at the sides as two pairs of wings, an anterior and a 

 posterior, more or less continuous ; but the roof of the cartila- 

 ginous brain-case in this region is incomplete. In this region 

 the basi-sphenoid and pre-sphenoid (b. s., p. s.) subsequently ossify 

 in the mid-ventral line, and the ali-sphenoids and orbito-sphenoids 

 (a. s., o. s.) are formed in the side wings. Anterior to this the 

 trabecular plate narrows and forms a median vertical lamina, 

 which further forward comes into relation with the nasal sacs, 

 curving over on either side to form their roof, or sending out 

 wings beneath to form their floor, or pioviding turbinal in- 

 growths. In this region the ethmoidal and turbinal bones are 

 subsequently developed. 



We have now to consider the part played by the visceral 

 arches. It will be remembered that in the tadpole there are 

 gill-slits leading outwards from the pharynx to the exterior. 

 Homologous gill-slits are formed in the fowl and the rabbit; 

 but in these organisms they never have a branchial function. 

 There are four such visceral clefts. In front of each, and 

 behind the last, is a visceral fold. Of the five visceral folds or 

 arches, the first lies just behind the mouth, and nearly meets its 

 fellow in the mid-line. This is the mandibular arch. It does 

 not remain simple, but bifurcates into two processes, a superior 

 maxillary process (Fig. 52, s.m.p.) in front of the mouth, and 

 an inferior maxillary process (i. m.p.) behind the mouth. The 

 second or hyoid arch is the strongest in the embryo. The 

 third, fourth, and fifth are progressively shorter and weaker. 



The first post-oral visceral cleft persists in the adult as the 

 Eustachian tube and tympanic cavity. The other three function 

 as branchial clefts in the frog, but are early obliterated in the 

 fowl and rabbit. 



The further history of the first two arches is remarkable and 

 important. In the superior maxillary process of the first a 

 cartilaginous palato-pterygoid bar is developed, in relation to 

 which the palatine and pterygoid bones are formed. In the 

 inferior maxillary process of the mandibular arch a cartilage 

 (Meckel's) is formed which constitutes the basis of the lower 

 jaw. In the frog its proximal portion forms the suspen- 

 11 



