380 



ORGAN SYSTEMS OF MAN 



opcntnq for 

 spinal cord 



openiriQ for 

 spinal mrvz 



Fig. 15-5. The spinal column is composed of interlocking 

 vertebrae that, taken together, form a sturdy, flex- 

 ible support for the entire body. The large openings 

 in the vertebrae form a bony canal in which the 

 delicate spinal cord is housed. Between the vertebrae 

 are paired openings through which the spinal nerves 

 pass. 



vertebrae of the thoracic region are rela- 

 tively immovable whereas those in the 

 lower back and neck region have consid- 

 erable amplitude of movement. This ar- 

 rangement allows for a large variety of 

 movements of the trunk, as evidenced by 

 the ballet dancer in action. 



Pairs of small openings between the 

 vertebrae provide exits for the spinal nerves 

 (Fig. 15-5). Each vertebra has a large 

 cylindrical passagev/ay, and these taken 

 together form the neural canal which 

 houses the nerve cord (Fig. 15-5). Five of 

 the lower sacral vertebrae are fused into a 

 solid bone, the sacrum, which joins the ilia 

 (singular — ilium) on the dorsal side, thus 

 securely attaching the pelvic girdle to tiie 

 spine. It needs to be a broad, secure attach- 

 ment because the whole upper body pivots 



at this point and the stress is considerable. 

 Unfortunately, the joint is not bone-to- 

 bone but via ligaments, and when it par- 

 tially gives way under unusual strain much 

 distress is caused. 



The spinal column terminates in sev- 

 eral (5 to 12) tiny, useless vertebrae, col- 

 lectively known as the coccyx. In many ver- 

 tebrates they give support to a functional 

 tail, but in man they are mere vestiges of 

 the past. Undoubtedly, far back in man's 

 early history, long before he was man, he 

 had a tail. It must be remembered that the 

 presence or absence of a tail means nothing 

 from an evolutionary point of view. The 

 bear and guinea pig are without tails, yet 

 they are no more related to each other than 

 either is to man. 



The ribs. The 12 ribs are attached to the 

 transverse processes and the centra on the 

 column side and 10 of them to the sternum 

 directly or indirectly on the ventral side. 

 These together form the thoracic basket, a 

 convenient enclosure for the vital organs 

 located in the chest region. It is interesting 

 to note that the number of ribs is not 

 always 12. The millions of chest x-rays taken 

 of soldiers in the last war brought to light 

 the fact that tliere is considerable variation, 

 ranging from 11 to 13, the latter number 

 beino; the most common variation. Inciden- 

 tally, the gorilla also possesses 13 ribs. 



The appendicular skeleton 



The remainder of the skeleton, consisting 

 of the appendages and their supports or 

 girdles, is called the appendicular skeleton. 

 The pectoral girdle to which the arms are 

 attached is located in the anterior region. 

 It consists of two clavicles (collar bones) 

 and two scapulas (shoulder blades); taken 

 together they form a triangular brace with 

 the arm hanging at the apex. Clavicles are 

 rudimentary or absent in most mammals, 

 but in the primates they are large, func- 

 tional bones. This difference is owing to 

 life in the trees, where brachiation was re- 

 sponsible for the development not only of 



