94 ELEMENTARY ANATOMY. [LESS. 



veloped (one in front of the other) on each side of the chorda 

 dorsalis, and which are the precursors of the vertebrae. 



A solid, flattened mass surrounds the anterior termination 

 of the chorda dorsalis, and forms the cartilaginous foundation 

 and precursor of the basilar part of the occipital bone, and 

 may be called the basi-cranial plate. 



Continuous with this on each side is a rounded mass of 

 cartilage which is the precursor of the petrous parts of the 

 temporal bone and of the lateral parts of the exoccipital ; and 

 the latter cartilages, growing up to meet together above, 

 enclose the foramen magnum with a cartilaginous ring. 



From the front of the basi-cranial plate two cartilaginous 

 rods, called trabeculce cranii?- extend forwards and down- 

 wards. These at first diverge, and then converge and meet, 

 thus leaving a space in which the pituitary fossa comes to be 

 placed. 



ati 



FIG. 93. DIAGRAM OF THE FORMATION OF THE SKULL, SEEN LATERALLY. 

 N, the anterior termination of the notochord in the cartilaginous occipital mass ; 

 EO, a lateral upward extension of that mass, forming the foundation of the 

 future lateral part of the occipital bone ; att, the auditory capsule ; a and o. the 

 cartilaginous beginnings of the future " greater" and "orbital" wings of the 

 sphenoid ; i, one of the trabeculie cranii running forwards to expand in the 

 ethmo-vomerine plate or foundation of the future median (ME) and lateral (LE) 

 ethmoids ; 2, the second (maxillary) arch running forwards to abut against the 

 ethmo-vomerine cartilage ; 3, the mandibular arth ; 4, the hyoidean arch. 



This prolongation forwards, after sending up on each side 

 two cartilaginous representatives of the sphenoidal wings, 

 forms, as has been indicated, a median plate (termed Ethmo- 

 vomerine), which sends downwards three other plates to form 

 the median and lateral ethmoids. 



The rest of the walls and roof of the skull are completed 

 by membrane only. 



Besides the trabeculas, other cartilaginous rods extend" 



1 From trabecula, the diminutive of trabes, a beam or rafter. 



