154 VERTEBRAL CAVITY. 



when these bones grow together, all of them are capable of 

 motion ; of which the first and second enjoy the largest share. 



The lower end of the fourth bone terminates in a rough point, 

 to which a cartilage is appended. 



To the sides of these bones of the os coccygis, the coccygaei 

 muscles, and part of the levatores ani, and of the glutaei maximi, 

 are fixed. 



The connexions of thes% bones hinder them from being 

 moved to either side ; and their motion backwards and for- 

 wards is much confined : yet, as their ligaments can be stretched 

 by a considerable force, it is of great advantage in the excretion 

 of the faeces alvinae, and much more in child-bearing, that these 

 bones should remain movable ; and the right management of 

 them, in delivering women, is very important. The mobility 

 of the os coccygis diminishing as people advance in age, 

 especially when its ligaments and cartilages have not been 

 kept flexible by being stretched, is, probably, one reason why 

 women, who are advanced in years before they marry, have gen- 

 erally difficult parturition. 



These bones serve to sustain the intestinum rectum ; and, 

 therefore, are curved forwards ; by which they are preserved, 

 as well as the muscles and teguments, from any injury when 

 sitting with the body inclined back. 



The Vertebral Cavity for containing the Spinal Marrow. 



The canal, formed by the foramina of the different vertebrae, 

 when these bones are placed in their natural order, extends 

 from the great occipital foramen to the end of the sacrum. Its 

 direction varies with the different curvatures of the spine, and 

 its figure and diameter are also very different in different 

 places. 



In the cervical vertebrae, it is largest, and nearly triangular 

 in form ; in the dorsal, it is much smaller and almost cylin- 

 drical ; in the lumbar, it is somewhat enlarged, and approaches 

 again to the triangular figure ; in the sacrum, it is broad, but flat, 

 and diminishes gradually, so as to assume the form of a long 

 triangle. 



