Visceral Arches. 7 



when, as is often the case, the cartilage becomes con- 

 verted into bone, the several pieces of which the osseous 

 skull consists are so grouped around these nerves and 

 sense organs that the bony cranium appears as if its 

 bones were arranged in a succession of segments. 

 These have been mistaken for true vertebral divisions, 

 but are really due to a secondary grouping of parts in 

 the course of growth, and are not primary morpho- 

 logical elements. Appended to the under or mouth 

 side of the cranium, and to the fore part of the 

 vertebral column, we find a series of lateral arches, 

 which unite below in the medial line, and thus 

 close in the sub- vertebral cavity in front To these 

 arches the name 'visceral arches/ is given ; and very 

 often between these arches there a r e slits opening 

 inwards ; these are called visceral slits. The number 

 of these arches varies in many vertebrates, but there 

 may be as many as ten or twelve. The foremost is in 

 front of the part of the skull which begins at the front 

 end of the notochord (for this structure does not in 

 craniotes extend beyond the region of the mid-brain), 

 and its two elements pass forwards in the middle 

 line to unite in front; to these the name cornua 

 trabeculcK is given. The second arch lies behind, 

 below, and a little outside the cornua trabeculse, and 

 forms part of the deeper or palatine portion of the 

 upper jaw in most vertebrates (or the whole upper 

 jaw in sharks) ; its lower end forms the lower jaw, or 

 parts thereof. The third or hyoid arch is that bony 

 system on which the tongue is based ; and the suc- 

 ceeding ones can be easily distinguished in fishes as 

 the arches of bones which bear the gills, but, except 



