128 



CERVICAL 



the lateral movements of the head are accomplished thereby, 

 whilst the upward and downward movements are performed by 



Fig. 108. Fig. 109. 



the play of the condyles of the occipital bone (fig. 90, 16) on the 

 broad concave articular surfaces of the atlas (fig. 108, 2). Fig. 

 108 shews the superior, and fig. 109 the inferior surface of this 

 vertebra. A firm ligament is stretched across the ring, dividing 

 it into two apertures ; the anterior hole (1) receives the tooth- 

 like process of the axis, the posterior hole (6) gives passage to 

 the spinal cord. The essential element of a vertebra is the 

 centrum, the next in constancy are the two neurapophyses, the 

 other elements undergo various phases of development. We 

 rarely find all the elements present in one vertebra ; some 

 are absent, others are rudimentary, and others expand into 

 disproportionate dimensions, in order to accomplish some 

 destined end. A typical vertebra with all its elements, presents 

 four channels disposed around the centrum; we find this 

 typical vertebra in the thorax of mammals, birds, and lizards. 

 Let us take, for example, the third, fourth, or fifth dorsal 

 vertebra of man (fig. 105) : the centrum (a, b) is broad, solid, 

 and slightly biconcave ; from its posterior part arise the two 

 neurapophyses (fig. 105, 7), which arch over and enclose the 

 spinal cord (6), and terminate in the neural spine (5) ; the two 

 transverse or parapophyses are seen at (4, 4) ; to the sides of the 

 centrum the dorsal ribs or two pleurapophyses are attached (fig. 

 1 24y,ihehtemapophyses are represented by the sternal cartilages, 

 which are united to the distal extremity of the ribs; the htemal 

 element is a broad flat bone, forming one of the segments of the 

 sternum ; these five elements unite to form one of the large hoops 

 of the thoracic cage (fig. 124), for enclosing and protecting 

 the heart and the great trunks of the vascular system ; the 

 lateral channels giving transit to the nerves and blood-vessels. 



