100 The Vertebrate Organ Systems 



Foramen : An opening in a bone for the passage of nerves and blood 



vessels. 

 Fossa: A depression. 



Process: A protuberance or projecting part of a bone. 



Suture : Line of union between bones of the skull, often irregular. 



Symphysis : Union of two bones by means of cartilage. 



THE AXIAL SKELETON 



As its name implies, the axial skeleton (Fig. 30) is really the 

 axis of the body. It is composed of the skull, the backbone, the ribs, 

 and the sternum. To it are attached the girdles and appendages of the 

 appendicular skeleton. In addition, it has the function of providing a 

 strong protective housing for the delicate brain and spinal cord. 



The axial skeleton is one of the chief distinguishing features of 

 the chordate phylum. All members of this phylum have some sort of 

 dorsal-supporting structure ; that is, all possess a dorsal cartilaginous 

 rod, the notochord, during some period of their development. This 

 cartilaginous rod is retained throughout life only among certain proto- 

 chordates and some low^er vertebrates. In the higher vertebrates, the 

 notochord is replaced by the vertebrae in a process of gradual growth 

 which squeezes out the cartilaginous elements of the notochord. In 

 the adults of these forms, the notochord remains only as cartilaginous 

 pads between the individual vertebrae. 



The Skull. — The most anterior part of the axial skeleton consists 

 of the skull (Fig. 31) which provides housing for the brain, support 

 for the jaws, and protection for the sense organs of smelling, seeing, 

 and hearing. 



Among the different vertebrates, the skull is variously developed. 

 In such forms as the more primitive fish there may be up to 180 differ- 

 ent bones in the skuU- In the changes that have taken place among 

 the vertebrates, the number of bones has been reduced by coalescing 

 of various elements. In man there are only 29 separate bones. 



The skull is derived from two separate sets of bones. One of 

 these, the inner, is laid down in cartilage, while the outer consists of 

 membrane bones. It is generally believed that the outer bony part 

 of the skull corresponds to the dermal scales of the ganoid fish such as 

 the gar. The inner skull is at first cartilaginous and is replaced by 

 bone. A cartilaginous skull has been retained only in a few fish 

 such as the shark. The dermal scales have sunk in to coalesce with the 

 cartilaginous portions in. other forms. 



