74 GENERAL BIOLOGY 



The maxilla and the premaxilla bear teeth. The lower jaw or 

 mandibular arch ( ), is made up of a pair of carti- 



laginous rods (Meckel's cartilages), enforced by a pair of dentary bones 

 ( ) and a pair of angulo-splenials ( ). 



The jaws themselves are attached to the cranium by an apparatus con- 

 sisting of squamosals ( ), pterygoids ( ) 

 and palatines ( ), the whole often known as a sus- 

 pensory apparatus, or a suspensorium ( ). These 

 bones, though attached to the cranium in the adult frog, are at first free 

 from it, being in reality the upper parts of what are called visceral 

 arches, which lie below the cranium. The second arch is called the 

 hyoid, and is quite rudimentary, only a small part of it being left in the 

 adult frog. In the higher forms such as man, this is a well-developed 

 V-shaped arch to which the tongue is attached, but in the frog it remains 

 only as a flat plate, partly bone and partly cartilage, so loosely attached 

 to the skull that it is quite easily, and one might add, usually, lost. It 

 lies directly beneath the larynx ( ) in the frog, giv- 

 ing this support and rigidity, being connected with the skull by liga- 

 ments ( ) only. 



In the young frog all parts of the skull are soft, but true bone forms 

 as development goes on. A part of the skull forms originally as carti- 

 lage, a material that is harder than membrane but softer than bone. 

 Mineral matter is deposited a little later in the cartilage, causing ossifi- 

 cation ( ) or true manufacture of bone. 



Bones such as the occipitals, parietals, pterygoids, and the mandi- 

 bles, formed from cartilage, are known as cartilaginous bones, the other 

 ones being manufactured first as membranes. Here, too, mineral matter 

 is laid down and the structures become hardened. Such bones as 

 frontals, parietals, parasphenoids, squamosals, nasals, vomers, pre- 

 maxillas, and maxilla are of the latter kind and are called membrane 

 bones. The projections at the posterior end of the skull where it con- 

 nects with the vertebral column are called occipital condyles ( ) ; 

 and the large opening directly between these through which the spinal 

 cord continues down through the bony canal of the spinal column is 

 called the foramen magnum ( ). 



THE VERTEBRAL COLUMN 



This consists of nine separate segments of bone (H and I, Fig. 20), 

 each known as a vertebra ( ), and a long platelike 



posterior extension, the urostyle ( ). Each verte- 



bra consists of a centrum and a neural arch ( ), the 



latter enclosing the neural foramen; on each side of all but the first 

 vertebra is found a transverse process, while all vertebrae possess a dor- 

 sal spine and a pair of smooth surfaces where each successive vertebra 

 rests upon the next following. These articulating processes are called 



