VERTEBRAL COLUMN. 91 



the sinuses bordering upon it. The head, though nearly ba- 

 lanced, has some preponderance in front, which is manifested 

 by its falling forwards whenever we sleep in the erect position, 

 or when the sudden suspension of life destroys the contraction 

 of the muscles on the back of the neck. 



In the lower orders of animals, the obliquity of the condyles, 

 their situation at one end of the head, and the great length of 

 the face, acting as a weight upon a long lever, have a continual 

 tendency to incline the head downwards ; which is only par- 

 tially counteracted by the largeness of the muscles and liga- 

 ments on the back of the neck. 



The horizontal direction of the condyles, and their location 

 near the centre of the base of the head, have arrested the atten- 

 tion of naturalists, and established for man characters distin- 

 guishing him, from all other animals, for facility in maintaining 

 the erect attitude. Bichat happily observes, that from this con- 

 formation result the following peculiarities in his organization : 



1. Less strength in the muscles of the neck than in quadrupeds; 



2. Less projection in the occipital bone, where the muscles are 

 inserted ; and, 3. An imperfect development of the ligamentum 

 nuchae. 



The thoracic and abdominal viscera, by being placed in front 

 of the spine, and having no counterpoise behind, have a conti- 

 nued tendency to bend it. This is only resisted by the mus- 

 cles which fill up the long gutter on either side of the spinous 

 processes, and are inserted into the ribs, the spinous and the 

 transverse processes. The lumbar vertebrae and the appertain- 

 ing muscles and ligaments, having an increased duty to per- 

 form, from the lowness of their position, and the variety of their 

 movements, become the soonest affecteM by fatigue and bodily 

 weakness; and therefore manifest sooner the' sensation of las- 

 situde, notwithstanding the augmented volume of the bodies 

 and processes of the vertebra), and of the muscular masses in- 

 serted into them. 



The mechanical arrangement of the spine permits it to per- 

 form the motions of flexion, extension, lateral bending, circum- 

 duction, and rotation. 



1. Flexion, or that posture in which the spine is bent for- 

 wards, is the most extensive of its movements : the general me- 



