CHAPTER VI. 

 THE BACK. 



The Spine. The trunk may be roughly 

 divided into the back, the chest or thorax, 

 the abdomen, and the pelvis. By the 

 back is denoted the spinal column with its 

 muscles, blood-vessels, etc., and the spinal 

 cord already described. The spine or 

 vertebral column, which serves the double 

 purpose of holding the body erect and of 

 protecting the cord, is usually about two 

 feet, two inches in length. In its course 

 there occur several curves, which serve to 

 give springiness and strength and, with 

 the intervertebral cartilages, to mitigate 

 the force of concussion from blows and 

 falls. The curve is convex forward in the 

 cervical region, convex backward in the 

 dorsal, forward in the lumbar, and back- 

 ward again in the sacral region. There is 

 most freedom of motion in the cervical 

 region. 



As is the case with the other bones, the 

 vertebrae are specially adapted in shape and 

 size to the needs they are called upon to 

 fill. Strength and flexibility, with a mini- 

 mum bulk, a channel for the cord, and 

 passages for the numerous nerves and 

 blood-vessels are some of the requirements 

 which, in combination, they meet to an 

 astonishing degree. They are thirty-three 

 in all, and are divided into groups according 

 Fio 31 _ The to the region in which they occur: seven cer- 

 spinai column, vical in the neck, twelve dorsal or thoracic, 

 ' five lumbar, five sacral, and four coccygeal. 

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