4 Mr Bettany, On the Primary Elements of the Shull. [Oct. 30, 



Professors Huxley and Parker liave for some years been led to 

 view the trabecidce cranii, tlie primary elements underlying the 

 base of the fore part of the brain, as not axial in character, but as 

 comparable in effect to the visceral arches, or mandibular, hyoidean, 

 and branchial series. In his Anatomy of Vertehrated Animals, 

 1871, p. 77, this view is alluded to by Prof. Huxley as a hypothesis 

 or speculation, which must not be placed on the same footing as 

 the doctrine of the segmentation of the skull. But without, as 

 far as I am able to discover, any full and formal exposition of the 

 evidence, this hypothesis has gradually been elevated almost to the 

 rank of a doctrine by the two writers named above, until in a 

 paper on Ceratodus by Prof. Huxley, in the Proceedings of the 

 Zoological Society for 1876, Part I., the trabeculse are classed with- 

 out reserve as pleural elements. The following appear to be some 

 of the chief considerations in favour of the view that the trabeculse 

 are visceral arches : 



1. Their downbent condition during the mesocephalic flexure, 

 when they necessarily appear more or less parallel to the visceral 

 arches behind. 



2. Their transitory distinctness in several types from the 

 parachordal elements which lay the foundation of the hinder part 

 of the skull. 



3. The fact that the notochord does not extend beyond the 

 pituitary body which is developed between the hinder regions of 

 the trabeculse. 



4. Their anterior junction in the ethmoidal region has some 

 analogy with the ventral junction or union between each pair of 

 visceral arches. 



5. Their more or less curved shape at first. 



6. The distribution of the orbitonasal division of the tri- 

 geminal nerve, which presents some features of resemblance to the 

 distribution of nerves on the visceral arches. 



Against the weight of these considerations it may be alleged, 

 that the tissue beneath the fore brain before the mesocephalic 

 flexure has the same claim to be called axial as the tissue beneath 

 the hinder part of the brain; that the fact of the mesocephalic 

 flexure carrying it into a different position does not make this 

 tissue other than axial ; and that it resumes the axial position 

 very early. The trabeculse in almost all cases become united with 

 the parachordals before uniting with each other, constituting a 

 sino'le bar on each side in the base of the brain. The significance 

 of the notochord and pituitary body is unknown ; but in several 

 instances (Axolotl, Dog-fish) the trabeculse acquire a distinct para- 

 chordal region which may be proportionately very extensive. As 

 development advances, the side walls of the brain-case chondrify 

 continuously with the trabeculse just as the occipital ring is formed 



