732 



DEVELOPMENT OF THE HEAD. 



cartilages. It is mainly produced in connection with the trabeculae 

 cranii, which contain between their separated limbs the pituitary fossa. 

 This part of the cranial basis contains no prolongation of the noto- 



Fig. 500. 



Fig. 530. — The Lower or Cartilaginous part op 

 THE Cranium op a Chick op thk Sixth Dat 

 (from Huxley). 



1, 1, cliorda dorsalis ; 2, the shaded portion here 

 and forwards is the cartilage of the base of the skull ; 

 at 2 the occipital part ; at 3 the prolongations of 

 cartilage into the anterior part of the skull called 

 traheculce cranii ; 4, the pituitary space ; 5, parts of 

 the labyrinth. 



chord ; it lies below the anterior encephalic 

 vesicle (thalamencephalon), and becomes 

 greatly modified in connection with the 

 expansion of the cerebral hemispheres and 

 primary ocular vesicles, and the develop- 

 ment of the nasal fossae and mouth, together with the other parts of 

 the face. 



The primary parts of the three principal sense organs, it may here be 

 stated, the nose, eye, and ear, formed in connection respectively with 

 the cerebral hemispheres, the thalamencephalon, and the third primary 

 vesicle, are interpolated between the rudimentary parts of the head as 

 follows, viz., 1. The nose between the frontal, intermaxillary and 

 ethmoid ; the eye between the frontal, sphenoid, ethmoid and maxillary ; 

 and the ear between the basi-occipital, exoccipital and alisphenoid. 

 "While the base of the cranium, to the extent already mentioned, is 



Fig. 531. Fig. 53L — View from below op the Carti- 



laginous Base of the Cranium with 

 its Ossific Centres in a Human Foetus 

 op about Five Months (from Huxley, slightly 

 altered). 



The bone is dotted to distinguish it from 

 the cartilage, which is shaded with lines. 

 1, the basilar part, 2, the condyloid or 

 lateral pvarts, and 3, 4, the tabular or superior 

 part of the occipital surrounding the foramen 

 magnum ; 5, centres of the pre-sphenoid on the 

 inside of the optic foramen ; 6, centres of the 

 post-sphenoid ; 7, centres of the lesser wings 

 or orbito-sphenoid ; 8, septal cartilage of the 

 nose ; 9 & 10, parts of the labyrinth. 



cartilaginous in its origin, the lateral 

 and upper walls are chiefly of mem- 

 branous formation, as in the squama 

 occipitis, the squamo-zygomatic of the 

 temporal, the parietal and the frontal bones. 



The trabeculai stretch forward to the anterior extremity of the head, 

 and maintain the foremost place as the seat of the nasal cartilages and 

 external apertures of the nose. Behind these the coalesced trabeculae 

 form a narrow cthmo-vomerine cartilage, the nasal septum, round the 



