5/0 THE TKAHECUL/E. 



vertical growth of this plate in the region of the orbit forms the 

 interorbital plate of Teleostei, Lacertilia and Aves (fig. 326, ps), 

 on the upper surface of which the front part of the brain rests. 

 The trabecular floor of the brain does not long remain simple. 

 Its sides grow vertically upwards, forming a lateral wall for the 

 brain, in which in the higher types two regions may be distin- 

 guished, viz. an alisphenoidal region (fig. 326, as) behind, 

 growing out from what is known as the basisphenoidal region 

 of the primitive trabeculae, and an orbitosphenoidal region in 

 front growing out from the presphenoidal region of the tra- 

 becula,\ These plates form at first a continuous lateral wall of 

 the cranium. At the front end of the brain they are continued 

 inwards, and more or less completely separate the true cranial 

 cavity from the nasal region in front. The region of the cartilage 

 forming the anterior boundary of the cranial cavity is known as 

 the lateral ethmoid region, and it is always perforated for the 

 passage of the olfactory nerves. 



The cartilaginous walls which grow up from the trabecular 

 floor of the cranium generally extend upwards so as to form a 

 roof, though almost always an imperfect roof, for the cranial 

 cavity. In the higher types, in Mammals more especially, this 

 roof can hardly be said to be formed at all. The region of the 

 trabeculae in front of the brain is the ethmoid region. The basal 

 part of this region forms an internasal plate, from which an 

 internasal septum may grow up (fig. 326). To its sides the 

 olfactory capsules are attached, and there are usually lateral 

 outgrowths in front forming the trabecular cornua, while from 

 the posterior part of the ethmoidal plate, forming the anterior 

 boundary of the cranial cavity, there often grows out a prefrontal 

 or lateral ethmoidal process. 



These and other processes growing out from the trabeculse have 

 occasionally been regarded as rudimentary praeoral branchial arches. I 

 have already stated it as my view that the existence of branchial arches 

 in this region is highly improbable, and I may add that the development 

 of these structures as outgrowths of the skull is in itself to my mind a nearly 

 conclusive argument against their being branchial arches, in that true 

 branchial arches hardly ever or perhaps never arise in this way. 



The sense capsules. The most important of these is the 

 auditory capsule, which, as we have seen, fuses intimately with 



