104 The Vertebrate Organ Systems 



by narrow passageways to the nasal cavity. Due to their small passage- 

 ways and lining of mucous membranes, they often become infected. 

 These spaces give resonance to the voice. 



At birth the skull has not completely ossified. The ossification 

 proceeds from the centers of the bones toward the edges, leaving areas 

 between bones which are only covered with membranes. These rela- 

 tively unprotected areas are known as fontanels, due to a fancied 

 resemblance of the pulsating blood vessels beneath to a fountain. 

 These fontanels disappear after birth. No doubt, they are a useful 

 adaptation allowing some squeezing of the head during its passage 

 through the narrow birth canal. The ossified portions of bone can 

 slip over one another without harming the brain below. The entire 

 skull is quite pliable for a period after birth. Many primitive tribes 

 took advantage of this and shaped the heads of infants into their idea 

 of improvement on nature. 



Where the skull bones finally join to one another, irregular su- 

 tures result. This zigzag joining gives great strength to the skull. 



The Backbone. — As mentioned previously, a dorsal-supporting 

 notochord is characteristic of all chordates. Except for the protochor- 

 dates and a few fish, the notochord is replaced by the individual verte- 

 brae that form the backbone. The cartilaginous notochord offers suffi- 

 cient support to small animals dwelling in water, but a terrestrial 

 chordate needs the greater support which the bony vertebrae provide. 



The primary function of the backbone is to give needed support 

 to the body. The development of the backbone has taken the form of 

 an arch. This form is clearly demonstrated in a four-footed animal, 

 such as the cat or dog, where the arched backbone is supported by 

 the paired legs. This type of construction gives great strength with 

 a minimum of bulk and weight. Engineers utilize this principle con- 

 stantly in constructing bridges to span our great rivers. The upright 

 posture of man imposes certain additional problems on this arched 

 column. The heavy weight of the skull is placed on the upended 

 column. Balance is achieved by curvatures in the region of the neck 

 and above the pelvis. These are the cervical and litmbar curvatures, jj 

 In the infant these curvatures are not present ; until the child has de- ■ 

 veloped them he is very awkward in his walking. 



Cartilage is a very flexible material, and those chordates with 

 notochords move through the water by sinuous movements of the body. 

 While allowing for great flexibility, it is clear that the notochord does 



