in.] THE CRAXIAL SKELETON 93 



In the substance of the frontal bone, just above the nasals 

 are, almost always, certain cavities filled with air called the 

 frontal sinuses. These communicate with the nasal fossa 

 below, through the middle turbinals. 



The body of the sphenoid and the maxillae also contain 

 large air-holding cavities, termed the sphenoidal and mav- 

 illary sinuses, and opening respectively beneath the upper 

 and middle turbinals into the nasal fossse. 



17. In DEVELOPMENT the first appearance of the future 

 skull is indicated by the expansion of the anterior end of 

 the primitive groove before spoken of. 



FIG 92. DIAGRAM OF THE FORMATION OF THE SKULL, SEEN FROM ABOVE. 



BO, the plate of cartilage representing the future body of the occipital bone ; N, 

 the anterior termination of the notochord ; an, the cartilaginous auditory 

 capsule containing the internal ear ; p, the space afterwards becoming the 

 pituitary fossa, and now enclosed on its two sides by two cartilaginous rods 

 the trabecitl<z cranii, which meet in front of it at(i) to form the ethmo- 

 vomerine plate from which the median (ME) and lateral (LE) ethmoids arise ; 2, 

 the second cartilaginous rod. or arch, forming the maxillary arch; 3, the 

 third arch, forming the mandibular arch (or lower jaw) ; 4, the fourth arch, 

 forming the hyoidean arch. 



This expansion becomes subdivided by two lateral con- 

 tractions (one in front of the other, on each side), so that 

 three rounded cavities are produced, one behind, one above, 

 and one in front of the anterior termination of the noto- 

 chord the primitive skull being sharply bent down just 

 in front of the anterior termination of the notochord. 



In the walls of these rounded expansions, or vesicles, no 

 quadrate thickenings occur similar to those which are de- 



