COMPARISON OF VERTEBRAE. 21 



In the vertebrae generally 



The arch and the body unite in the 3rd year. 



The vertebra and its epiphyses unite in the 25th year or later. 

 In the atlas 



Ossification of the anterior arch appears in the 1st year. 



The two centres of the posterior arch unite in the 3rd year. 



The anterior and posterior arches unite in the 5th or 6th year. 

 In the axis 



The centre of the body and (a little later) the centres of the odontoid process 

 appear in the 6th month. 



The body and odontoid process unite in the 3rd year. 

 In the sacrum 



The additional lateral centres appear from the 6th to the 8th month. 



The lateral epiphysal plates appear from the 18th to the 20th year. 



The body and arch unite in the 5th vertebra in the 2nd year. 



The body and arch unite in the 1st vertebra in the 5th or 6th year. 



The lower vertebrae unite with one another in the 18th year. 



The upper vertebrae unite with one another in the 25th year or later. 

 In the coccyx- 

 Ossification of the 1st vertebra appears about the time of birth. 



Ossification of the 2nd vertebra appears from the 5th to the 10th year. 



Ossification of the 3rd vertebra appears from the 10th to the 15th year. 



Ossification of the 4th vertebra appears from the 15th to the 20th year. 



THEORETICAL COMPARISON OF THE VERTEBRA. 



The study of the correspondence of parts in different vertebrae, or of their homology, 

 involves complicated questions, which cannot be competently discussed without refer- 

 ence to the various forms found throughout the vertebrate classes of animals. But 

 there are some points connected with this subject to which the attention of the 

 student of human anatomy may here be briefly directed. 



In considering the serial relations of the bodies of the vertebrae, it is necessary to 

 distinguish the masses formed round the chorda dorsalis the centra from the angular 

 portions derived by ossification from the arch. The only peculiarities in the series of 

 centra are to be found at or near the extremities of the column. One of these, of less 

 note, is presented by the first coccygeal vertebra, the cornua and lateral projections of 

 which, although ranged in series with parts which in the preceding vertebrae are derived 

 from the osseous nuclei of the arch, are, nevertheless, ossified from the centrum. The 

 other peculiarity presented by the atlas and axis is of a more remarkable kind. 

 The odontoid process of the axis is the true centrum of the atlas, disguised by 

 remaining distinct from the rest of the vertebra to which it belongs, and becoming 

 adherent to the centrum of the succeeding vertebra. The main proofs that this view- 

 is correct are, 1st, that the odontoid process originates in cartilage distinct from the 

 proper centrum of the axis ; 2nd, that the remains of the chorda dorsalis have been 

 traced through the odontoid process to the occipital bone ; and 3rd, that in early life, 

 and in certain animals (especially turtles), the odontoid process is similar in appearance 

 to the succeeding centra. The anterior nucleus of the atlas must, therefore, be regarded 

 as a precentral or subcentral structure. (Rathke, " Entwicklungsgeschichte der 

 Matter," p. 120; and "Entwickl. der Schildkrb'ten," p. 77.) It is proper to observe that 

 both sets of the articular surfaces of the atlas and the superior pair belonging to the 

 axis are placed upon those parts which in succeeding vertebrae form the angles of the 

 bodies ; these articular surfaces, therefore, correspond in position, not with the smooth 

 facets of the articulating processes, but rather with those for the heads of the ribs in the 

 dorsal vertebrae. This is seen not only from their position in front of the trunks of 

 the spinal nerves, and from the superior articular surfaces of the axis actually abutting 

 on the centrum, but also from the condition of parts in many animals. Thus, in 

 birds and reptiles the lateral masses and anterior nucleus of the atlas unite to form a 

 mesial disc, which articulates with the body of the axis in front of the odontoid 

 process and with a single condyle on the occipital bone, while at the same time the 

 axis and atlas are likewise connected by articulating processes placed behind the 

 nerves and in series with those which follow. 



