90 Elementary Zoology. 



arches — structures which were all originally bars of cartilage 

 for the strengthening and support of the branchiae. 



4. Of the labial cartilages. 



5. Of certain tnembrane bones ossified in the skin generally, 

 or limited to the head region. 



To understand how these various and quite different tracts 

 of cartilage or bone combine to form the solid whole that 

 we term the skull, it will be convenient to briefly trace the 

 development. 



1. The brain-case in its early condition consists of a pair 

 of stiff rods situated anteriorly, and known as the trabecules 

 cranii ; these lie in front of the notochord, and become fused 

 with each other anteriorly between the nasal sacs, again 

 diverging in front of the area of fusion to form a plate on 

 each side, which dips into and supports the upper lip. As the 

 embryo grows the trabeculse grow backwards and come into 

 contact with a pair of rods lying on either side of the notochord, 

 the parachordal cartilages. 



The state of affairs arrived at is illustrated in Fig. 37. 

 All these cartilages constitute the brain-case, or cranium, and 

 the brain lies upon the platform thus formed. Ultimately the 

 walls of the brain-case are formed by the growing up of the 

 sides of these cartilages, which come to more or less enclose 

 the brain, fontanelles being left dorsally. In the frog it so 

 happens that the trabeculse cranii are formed before the para- 

 chorials, but this is not the rule among the Vertebrata. The 

 separate bones, which are finally formed by the ossification of 

 the cranium, will be mentioned later. We may next consider — 



2. The capsules of the sense organs. Of these, the auditory 

 capsules alone have an important share in the building up of the 

 skull. Quite independent of the parachordal cartilages, but 

 abutting upon them, is a roundish mass of cartilage on either 

 side, which contains the auditory organ. This soon becomes 

 continuous with the side walls of the skull, and helps in the 

 formation of its side, and even its dorsal walls. The capsule of 

 the eye has practically no share in the building up of the skull. 

 The sclerotic coat of the eye is cartilaginous in the lower 

 vertebrates, and in birds becomes ossified into a ring of bone- 



