142 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



body or centrum occupies the space previously filled by notochordal cells and 

 is so shaped that it fits closely to its neighboring centra or to the intervertebral 

 discs. Dorsal to the centrum is the neural arch; fitted closely together the 

 neural arches form the bony canal in which the nerve cord is enclosed. Each 

 vertebra has particular areas, knobs, and edges, the attachment places of 

 ligaments and tendons of muscles that bind one vertebra to another, as well 

 as surfaces where the centra are pressed against the intervertebral discs. The 

 thoracic vertebrae have special hollows where the ribs articulate. 



The joints between the vertebrae have only limited freedom of motion, yet 



Fig, 9.6. A section through articulated 

 human vertebrae, showing one of the 

 intervertebral disks that separate the suc- 

 cessive vertebrae; / and 2, ends of circu- 

 lar fibers; 3, central cushion of cartilage 

 (nucleus pulposus). (Courtesy, Quain's 

 Elements of Anatomy, ed. 1 1. New York, 

 Longmans, Green & Co., 1915.) 



the backbone, like the spring from a curtain roll, can be bent backward, for- 

 ward, or sideways and swung back into place (Fig. 9.7). A cat's back can 

 take a high curve in a split second, and that of a bucking bronco outdoes the 

 cat in curves; it lifts a cowboy and is just as fast. A snake coils and twists; a 

 kitten sleeps in a ball; an owl rotates its head until it looks directly behind 

 itself; and human acrobats are close competitors, yet the vertebrae stay in their 

 places. 



Joints. In endoskeletons the muscles and ligaments are fastened to the outer 

 surfaces of the cartilages and bones. Some joints are immovable, such as those 

 in the cranium, little noticed except in very young infants in which they have 

 not grown together. Among the familiar types of movable joints are (Fig. 9.8) : 

 (1) hinge joints, such as those that are worked hard in typewriting; (2) ball- 

 and-socket such as the hip joint in which the head of the femur fits into the 

 pelvic girdle, a joint that is highly important in tap dancing, as well as in 

 walking and sitting and rising; (3) rotating joints in which the radius of the 

 human forearm shifts on its axis across the ulna as when the hand turns a door- 

 knob; and (4) pivotal joints that rock one upon another, such as the im- 

 portant "yes and no" joints, in action as the skull rocks upon the first vertebra 

 (atlas) when we nod "yes"; the atlas revolves upon the vertebra behind it 

 (axis) when we shake our heads "no." 



In every typical free-moving joint the ends of the bones are held together 

 by sheets of tough connective tissue, the ligaments that enclose the joint in a 



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