4f,S 



Til K TRABECULJS. 



C V 1 



with the periotic cartilaginous capsules, and the occipital ting above 

 usua'ly spreads forward to form a roof for the part of the brain 

 between these capsules. In the higher Vertebrates the periotic 

 cartilages may be developed continuously with the basilar plate 



(tig. 325). 



The trabeculse. The trabeculae, 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 trabeculae 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. 



The nature of the trabecula? has 

 been much disputed by morpholo- 

 gists. 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, coin- 

 cides 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 truhecula? must 

 I think be admitted, I see no 

 reason against supposing that tlie 

 trabecula? 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 liind-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 primitive 

 extension of the base of the fore- 

 brain through the pituitary space is 



nc 



FIG. 325. VIEW FROM ABOVE OF THE 

 INVESTING MASS AND OF THE TKABECUL.E 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. 



cr 1. cerebral vesicle (sliced off); e. 

 eye ; nc. notochord ; jr. 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 ; Ig. expanded 

 anterior end of the investing mass ; pts. 

 pituitary space ; tr. trabeculae. The re- 

 ference line tr. has been accidentally 

 made to end a little short of the cartilage. 



