138 



HUMAN ANATOMY. 



Synovial joints, the shapes of which are described with the bones, exist be- 

 tween the occipital bone and the atlas and between the atlas and the axis. The 

 capsule of the upper joint is very thick, especially behind, where it is continuous 

 with the posterior occipito-atloid ligament. The capsule surrounding the articular 

 surfaces of the atlas and axis is strengthened posteriorly by a bundle running upward 

 and outward from the axis. 



Fig. 169. 



i'osterior tubercle of atlas 

 pinal cord 



Posterior burs 



Transverse process of 

 atlas 



N'ertebral artery cut obliquely 

 Dura 



Apparatus ligamentosus 



Vertebral artery cut in 

 transverse foramen 



Anterior bursa Transverse ligament 

 Anterior tubercle of atlas 



Section of odontoid process 

 Transverse section of spine passing through atlas and odontoid process. 



THE SPINE AS A WHOLE. 



Anterior Aspect (Fig. 170). — The bodies enlarge, in the main, regularly 

 from above downward. This progression is interrupted only by a slight decrease 

 from the first to the fourth thoracic. In the cervical region the origin of the costal 

 elements from the sides of the bodies gives the latter a false appearance of breadth. 

 The middle of the thoracic region is particularly prominent in front, owing in part 

 to the aortic depression on the left. A slight curve to the right in this region is 

 generally seen ; it is probably attributable to this cause. 



Posterior Aspect (Fig. 170). — A deep gutter extends on each side of the 

 spinous processes, bounded externally in the neck and loins by the articular pro- 

 cesses and in the back by the transverse. In the latter region the spines which are 

 subcutaneous are often deflected from the median line, and may be arranged in zig- 

 zag. The laminae completely close the spinal canal in the convex thoracic and 

 sacral regions, while it is left open in the neck and loins, except during extension of 

 the former. 



Lateral Aspect (Fig. 171). — The profile view shows best of all the increase in 

 the importance of the bodies from above downward, and coincidently with this the 

 gradual moving backward of the intervertebral foramina. These increase greatly in 

 size from the lower part of the thoracic region. 



The Curves. — The curve of the spine is necessarily an arbitrary one, since it 

 varies not only in individuals and according to age, sex, and occupation, but also 

 with position and the time of day, being longer when lying than standing, and after 

 a night's rest than after a day's work. The difference occasioned by position occurs 

 especially in youth, when it may amount to half an inch or more. It is of little 

 consequence after middle age. Bearing these variations in mind, the following guide 

 to the curve, suggested by Humphry, may be accepted : a line dropped from the 

 middle of the odontoid process passes through the middle of the body of the second 

 thoracic, that of the twelfth thoracic, and the anterior inferior angle of the fifth 

 lumbar. Henle divides the spine into four quarters ; and although this method has 

 the defect of using the unreliable pelvic section, it very often proves remarkably 

 correct. Thus, if we continue Humphry's line to the level of the tip of the coccyx, 

 the middle point is opposite the eleventh thoracic, the end of the first quarter oppo- 

 site the lower border of the third thoracic, and that of the third quarter opposite 

 the lower edge of the fourth lumbar. 



The development of the curves can hardly be said to have begun at birth. At 



