438 



MUSCULAR MOTION. 



.they have a strong ligament posterior cervical or ligamentum nuclise. 

 (N, Fig. 189,) which extends from the spinous processes of the ver- 

 tebrae to the occiput, and aids in supporting the head. 



Fig. 189. 



Ligamentum Nuchae. 



The vertebral column supports the head, and transmits the weight 

 to its lower extremity. The tendency of the column is to bear for- 

 wards ; the upper limbs, neck, thorax with its contents, the greater 

 part of the contents of the abdomen, and the head itself, by reason of 

 its tendency to fall forwards, either directly or indirectly exert their 

 weight upon it. Hence the necessity for its great firmness and solidity, 

 which are readily appreciated, if we examine the mode of junction of 

 the different vertebrae, with the strong, ligamentous bands connecting 

 them, the whole having the form of a pyramid, whose base rests upon 

 the sacrum, with three curvatures in opposite directions, which give it 

 more resistance than if it were straight, and enable it to support very 

 heavy burdens in addition to the weight of the organs pressing upon 

 it. (Fig. 5, p. 77.) The tendency of the spine to fall forward is resisted 

 by the extensor muscles, which fill the vertebral fossae or gutters sacro- 

 lumbalis, longissimus dorsi, multifidus spinae, &c. and pass from the 



sacrum to the lower vertebrae of the spine, 

 and from the lower to the upper. Each 

 vertebra, in this action, constitutes a lever 

 of the first kind ; the fulcrum of which is in 

 the intervertebral cartilage; the power in 

 the ribs, and other parts that draw the body 

 forwards ; and the resistance in the muscles 

 attached to the spinous and transverse pro- 

 cesses. 



The vertebral column, regarded as a 

 whole, may be considered a lever of the 

 third kind ; the fulcrum of which is in the 



Lateral View of a Dorsal Vertebra. uni <>n between the last lumbar vertebra and 

 i. Body. G. spinous process. 7. sacrum, the power in the parts drawing the 

 Extremity of transverse process, s. spine forward, and the resistance in the 



Superior articular processes. 9. In- 1 r> , i i i T , i 



ferior articular processes. mUSCleS of the back. It IS On the 



Fig. 190. 



