The trabeculæ. The trabeculæ, 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 ([fig. 323]), 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 trabeculæ would probably seem to indicate, as pointed out by Gegenbaur, that these parts, in spite of their similarity to the parachordals, have not the same morphological significance.
Fig. 325. View From above of the investing mass and of the trabeculæ of a chick on the fourth day of incubation. (After Parker.)
In order to shew this, the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading.
cv 1. cerebral vesicle (sliced off); e. eye; nc. notochord; iv. investing mass; 9. foramen for the exit of the ninth nerve; cl. cochlea; hsc. horizontal semicircular canal; q. quadrate; 5. notch for the passage of the fifth nerve; lg. expanded anterior end of the investing mass; pts. pituitary space; tr. trabeculæ. The reference line tr. has been accidentally made to end a little short of the cartilage.
The nature of the trabeculæ has been much disputed by morphologists. The view that they cannot be regarded as the anterior section of the vertebral axis is supported by the consideration that the forward limit of the primitive skeletal axis, as marked by the notochord, coincides exactly with the distinction we have found it necessary to recognise, on entirely independent grounds, between the fore-brain, and the remainder of the nervous axis. But while this distinction between the parachordals and the trabeculæ must I think be admitted, I see no reason against supposing that the trabeculæ 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 trabeculæ 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 primitive 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 trabeculæ arise quite independently of the parachordals, though the two sets of elements soon unite; while in Birds ([fig. 325]) and Mammals the parachordals and trabeculæ are formed as a continuous whole. The junction between the trabeculæ and parachordals becomes marked by a cartilaginous ridge known as the posterior clinoid.
Fig. 326. Side view of the cartilaginous cranium of a Fowl on the seventh day of incubation. (After Parker.)
pn. prenasal cartilage; aln. alinasal cartilage; ale. aliethmoid; immediately below this is the aliseptal cartilage. eth. ethmoid; pp. pars plana; ps. presphenoid or interorbital; pa. palatine; pg. pterygoid; z. optic nerve; as. alisphenoid; q. quadrate; st. stapes; fr. fenestra rotunda; hso. horizontal semicircular canal; psc. posterior vertical semicircular canal: both the anterior and the posterior semicircular canals are seen shining through the cartilage. so. supraoccipital; eo. exoccipital; oc. occipital condyle; nc. notochord; mk. Meckel’s cartilage; ch. ceratohyal; bh. basihyal; cbr. and ebr. ceratobranchial; bbr. basibranchial.