224 



THE ARTICULATIONS 



i The apical ligament is tightened by extension and relaxed by flexion or nodding; the alar 1 

 1 ligaments not only limit the rotatory movements of the head and atlas upon the epistropheus, ' 



but by binding the occiput to the pivot, round which rotation occurs, they steady the headj 

 -^nd prevent its undue lateral inclination upon the vertebral column. (See Transverse 



Ligament, p. 222.) 



r By experiments, it has been proved that the head, when placed so that the orbits look a\ 

 I little upward, is poised upon the occipital condyles in a line drawn a little in front of their \ 



f' middle; the amount of elevation varies slightly in different cases, but the balance is always to 1 

 be obtained in the human body — it is one of the characteristics of the human figure. It serves \ 

 to maintain the head erect without undue muscular eflfort, or a strong ligamentum nuchae ■ 

 and prominent dorsal spines such as are seen in the lower animals. Disturb this balance, and 

 let the muscles cease to act, the head will either drop forward or backward according as the 

 centre of gravity is in front or behind the balance line. The ligaments which pass over the dens 

 to the occiput are not quite tight when the head is erect, and only become so when the head is 

 flexed; if this were not so, no flexion would be allowed; thus, muscular action, and nQt liga- 

 mentous tension, isemoloved to s teady th e head in the erect positioQ.. It is through the com- 

 bination of the joints of the atlas and epistropheus, and occiput and epistropheus (consisting of 

 two pairs of joints placed symmetrically on either side of the median line, while through the 

 median line there passes a pivot, also with a pair of joints), that the head enjoys such freedom 

 and celerity of action, remarkable strength, and almost absolute seciu-ity against violence, 

 which could only be obtained by a ball-and socket joint; but the ordinary ball-and-socket 

 joints are too prone to dislocations by even moderate twists to be reliable enough when the 

 life of the individual depends on the perfection of the articulation: hence the importance/ 

 •tof this combination of joints. ""^^ 



Fig. 263. — Coronal Section of the Vertebral Column and the Occipital Bone 



TO SHOW Ligaments. 

 (The tectorial membrane (1), though shown as a distinct stratum, is really the deeper part of 

 the posterior longitudinal ligament (2). The upper ends have been reflected 

 upward, the lower downward. Viewed from behind.) 



Vertical portion of crucial 



ligament 

 Apical dental ligament 



Alar ligaments 



Transverse portion of 

 crucial ligament 



Accessory band of atlanto 

 epistrophic capsules 



Atlanto-epistrophic joint 



Tectorial membrane 

 Posterior longitudinal ligament 



THE ARTICULATIONS OF THE TRUNK 



These may be divided into the following sets : — 



1. Those of the vertebral column. Joints and ligaments connecting; 



{d) The spinous processes, 

 (e) The transverse processes. 



(a) The bodies. 



(6) The articular processes. 



(c) The lamina?. 



2. Vertebral column with the pelvis. 



3. Pelvis. 



(a) Sacro-iliac 



(6) Sacro-coccygeal. 



I 



(c) Intorcoccygcal. 

 {d) Symphysis pubis. 



