152 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



take place. The atlanto-occipital joint, which separates the atlas 

 and the last occipital segment is of the same nature. The atlas 

 has neither the upper nor the lower articular processes of the other 

 vertebrae. Hence the 1st and 2nd cervical nerves appear to issue 

 behind the articular processes. At one time the single median 



-occip. 



-artic. 



-1st neural arch 



-artic. 



~2nd neural arch 



-artic. 



~3rd neural arch 



- 3rd centrum 



Fig. 122. — The nature of the Atlanto-axio-occipital Articulations. 



occipital condyle seen in birds and reptiles was regarded as very 

 different in nature from the double condyles of mammals. 

 Recently Symington has shown that in the lowest mammals 

 (monotremes), the occipital condyles are fused in the middle line, 

 and that foetal mammals show approximations to this condition. 

 In the human skull a remnant of this median fusion of the 

 condyles is frequently seen on the anterior margin of the foramen 

 magnum ; it is named the third or median occipital condyle. 

 Sometimes the atlas is partly fused with or imperfectly separated 

 from the occipital bone. This seems to be a further manifestation 

 of the process which has led to four segments, which were at one 

 time free body segments, being fused together to form the occipital 

 part of the skull. 



The Eibs are developed in the septa between the dorsal 

 primitive segments. At their vertebral ends they come in 

 contact with the vertebral bow (Fig. 118 B). In lower vertebrates 

 (birds, reptiles, etc.) each rib has two heads, a dorsal and ventral 

 (Fig. 123). The tuberosity of the human rib represents the 

 dorsal head ; the ventral head is well developed in man, as in 

 mammals generally. The rib articulates with the neural arches 

 only (Fig. 119 D). 



Vestigial Ribs. — Although the ribs are only fully developed 

 in the dorsal region, yet a representative — a costal element — is 



