4i6 CHORD AT A. 



or parachordals, soon fuse above and below the noto- 

 chord to form the basis cranii. The trabeculae then meet 

 in front under the fore-brain to form a median plate, 

 called the ethnio-nasal septum. All three pairs of sense- 

 organs now acquire cartilaginous sense-capsules, which, with 

 the exception of that of the eye (or sclerotic) fuse on to the 

 primitive cartilaginous cranium, or basal plate, formed by 

 the trabeculae and parachordals. The basal plate then 

 grows up on either side to enclose the brain. The edges 

 meet dorsally in the occipital region and also forwards in 

 the ethmoid region. Thus is formed the cartilaginous or 

 chondro-cranium. 



In the sharks and skates this condition remains, but 

 in Amphibia a number of bones are added, and in the 

 higher classes the bones almost completely replace the 

 cartilage, forming a complete osteo-cranium. This osteo- 

 cranium is produced partly by membrane-bones, which sink 

 in, and partly by cartilage-bones. The bones of the osteo- 

 cranium are arranged more or less in rings, a system which 

 gave rise, in the hands of Goethe, Oken and Owen, to' the 

 beautiful vertebral theory of the skull. The hindmost ring 

 is the occipital, with a basioccipital, two exoccipitals and a 

 supraoccipital. The second ring is the sphenoid, with a 

 basi sphenoid, two alisphenoids send a pair cA parietals. In 

 front of this is the presphenoid ritig, w ith presphenoid (at 

 base), paired orbitosphenoids and a pair oi frontals. The 

 ethmoid ring completes the front-end with a mesethmoid 

 and nasals. Between the occipital and sphenoid rings are 

 the periotic, a bony capsule of the ear,^ and the large 

 squamosal A Connected with the orbit, and lying at the 

 anterior corner of it, is the small lacrymal. % Lastly, immedi- 

 ately below the mesethmoid, in the roof of the mouth, are 

 the vomers, unpaired in mammals, and the parasphenoid. 



We have already seen that the skull is, in the course of its develop- 

 ment, preformed in membrane, and the greater part of it in cartilage- 

 The cartilage is then gradually replaced by bone, a stronger and harder 

 substance, by the process of ossification described above. If ossification 



* The periotic may be represented by as many as five separate otic bones, as in 

 the cod (p. 336). ^^ _ . . . " 



t The "temporal" bone of human anatomy is the fused periotic, tympanic and •' 

 squamosal. 



X The lacrymal is one of a series of circumorbital bones {c/. cod). 



