VIII.] THE TRABECULSE. 239 



The trabeculae. The trabeculse, so far as their 

 mere anatomical relations are concerned, play the same 

 part in forming the floor for the front cerebral vesicle 

 as the parachordals for the mid- and hind-brains. They 

 differ however from the parachordals in one important 

 feature, viz. that, except at their hinder end, they do 

 not embrace between them the notochord. 



The notochord constitutes, as we have seen, the 

 primitive axial skeleton of the body, and its absence in 

 the greater part of the region of the trabeculse would 

 probably seem to indicate, as pointed out by Gegen- 

 baur, that these parts, in spite of their similarity to 

 the parachordals, have not the same morphological 

 significance. 



While this distinction between the parachordals and the 

 trabeculse must be admitted, there seems to be no reason against 

 supposing that the trabeculse may be plates developed to support 

 the floor of the fore-brain, for the same physiological reasons 

 that the parachordals have become formed at the sides of the 

 notochord to support the floor of the hind-brain. By some 

 anatomists the trabeculse have been held to be a pair of branchial 

 bars ; but this view has now been generally given up. They 

 have also been regarded as equivalent to a complete pair of 

 neural arches enveloping the front end of the brain. The primi- 

 tive extension of the base of the fore-brain through the pituitary 

 space is an argument, not without force, which has been appealed 

 to in support of this view. 



In the majority of the lower forms the trabeculse 

 arise quite independently of the parachordals, though 

 the two sets of elements soon unite ; while in Birds 

 (Fig. 76) and Mammals the parachordals and trabeculse 

 are formed as a continuous whole. The junction be- 



