228 COMPARATIVE ANATOMY 



by its embryology. Both prove it to be a complex formed by the union 

 of diverse elements, capsules that contain the sense organs, and supports 

 for the gills, underpinning and protective covering for the brain, while 

 in the occipital region vertebrae appear to have fused with the brain 

 case. The primordial cartilaginous cranium of the human embryo arises 

 as a pair of parachordal cartilages and a pair of prechordal cartilages or 

 trabeculae. These fuse together; later they combine with two pairs of 

 sensory capsules, the olfactory and the auditory. This formation of the 

 cartilaginous basis of the cranium, the chondrocranium, takes place 

 during the second month of intra-uterine life. Ossification from separate 

 centers begins with the third. See Fig. 173. 



Evidence from comparative anatomy proves that the bones of the 

 human skull correspond to a much larger number of separate bones which 

 appear in the fishes and have been progressively reduced by fusion with 

 one another during the course of evolution. This also is borne out by 

 the development of the human cranium. 



Ossification of the occipital bone, for example, begins in four centers 

 corresponding with the basioccipital, the paired exoccipitals with their 

 condyles, and a supraoccipital of lower vertebrates. To these are later 

 added a membranous interparietal. Ossification of the occipital begins 

 in the third month but is not completed until the seventh year. 



The development of the sphenoid bone is even more complex, no fewer 

 than ten centers of ossification are recognized. Six of these arise in the 

 body of the bone and four more in the two paired wings. Membrane bone 

 is added both to the pterygoid processes and to the great wings. Fusion 

 of the separate elements is completed before the second year. 



Ossification of the ethmoid remains throughout life incomplete. 

 Three centers of ossification corresponding to the pro-, epi-, and opisthotic 

 bones of lower vertebrates develop in the otic capsule and help to form 

 the petrosal and mastoid portions of the temporal bone. The styloid 

 process of the temporal is an ossified portion of the hyoid cartilage which 

 fuses with the temporal. The squamous portion of the temporal is mem- 

 branous in origin. An outgrowth of the epithelium of the middle ear 

 penetrates the mastoid process to form a cavity or antrum. 



The rest of the cranium is membrane bone. Because the roofing 

 bones of the brain case ossify slowly and expand from centers, uncovered 

 regions or fontanelles persist for some months after birth as "soft spots" 

 between the frontal, parietal, and occipital bones. 



Evolution of the Visceral Skeleton. Evidence from both comparative 

 anatomy and embryology indicates that the upper and lower jaws, the 

 hyoid bone, the ear bones, and the laryngeal cartilages of man have evolved 

 from the skeletal gill supports of primitive fishes. 



