240 THE FIFTH DAY. [CHAP. 



tween the trabeculse and parachordals becomes marked 

 by a cartilagmous ridge known as the posterior cUnoid. 

 The trabeculae are somewhat lyre-shaped, meeting in 

 front and behind, and leaving a large pituitary space 

 between their middle parts (Fig. 76). Into this space 

 there primitively projects the whole base of the fore-brain, 

 but the space itself gradually becomes narrowed, till it 

 usually contains only the pituitary body. The carotid 

 arteries pass through it in the embryo ; but it ceases to 

 be perforated in the adult. The trabeculae soon unite 

 together, both in front and behind, and form a complete 

 plate underneath the fore-brain, ending in two horns in 

 the interior of the fronto-nasal process. A special ver- 

 tical growth of this plate in the region of the orbit 

 forms the interorbital plate (Fig. 78, 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 two regions may 

 be distinguished, viz. an alisphenoidal region (Fig. 78, 

 as.) behind, growing out from what is known as the 

 hasisphenoidal region of the primitive trabeculse, and 

 an orhitosphenoidal region in front growing out from 

 the presphenoidal region of the trabeculse. These 

 plates form at first on each side 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 trabeculse in 

 front of the brain is the ethmoidal region ; it forms the 

 anterior boundary of the cranial cavity. The basal part 

 of this region forms an intemasal plate, from which an 



