64 



SPECIAL ANATOMY OF THE SKELETON 



By 4 primary centres. 



2 for body (8th week). 



1 for each lamina (6th week). 

 FIG. 28. Development of a vertebra. 



By 2 additional plates. 



}- -1 for upper surface ' 

 of body, 



1 for under surface 

 of body, 



FIG. 29 

 By It secondary centres. 



21 years. 



arch, at birth, is altogether cartilaginous, and in this a separate nucleus appears about the end 

 of the first year after birth, and, extending laterally, joins the neural processes in front of the pedi- 

 cles. Sometimes there are two centres developed in the cartilage, one on either side of the median 



line, which join to form a single mass. 

 And occasionally there is no separate 

 centre, but the anterior arch is formed 

 by the gradual extension forward and 

 ultimate junction of the two neural pro- 

 cesses. 



Axis. The axis (Fig. 32) is developed 

 by seven centres, five primary and lico 

 secondary. The body and arch of this 

 bone are formed in the same manner as 

 the corresponding parts in the other 

 vertebrae: one centre (or two, which 

 speedily coalesce) for the lower part of 

 the body, and one for each lamina. 

 The centres for the laminae appear 

 about the seventh or eighth week, that 

 for the body about the fourth month. 

 The odontoid process consists originally 

 of an extension upward of the cartilagi- 

 nous mass in which the lower part of the 

 body is formed. At about the sixth 

 month of fetal life two centres make 

 their appearance in the base of this pro- 

 cess; they are placed laterally, and join 

 before birth to form a conical bilobed 

 mass deeply cleft above; the interval be- 

 tween the cleft and the summit of the 

 process is formed by a wedge-shaped 

 piece of cartilage, the base of the process 

 being separated from the body by a car- 

 tilaginous interval, which gradually be- 

 comes ossified at its circumference, but 

 remains cartilaginous in its centre until 

 advanced age. Finally, the apex of the 

 odontoid process has a separate (second- 

 ary) centre, which appears in the second 

 year and joins about the twelfth year. 

 In addition to these there is a secondary 

 centre for a thin epiphyseal plate on the 

 under surface of the body of the bone. 



Seventh Cervical. The anterior or 

 costal part of the transverse process of the 

 seventh cervical is developed from a 

 separate osseous centre at about the 

 sixth month of fetal life, and joins the 

 body and posterior division of the trans- 

 verse process between the fifth and sixth 

 years. Sometimes this process continues 

 as a separate piece, and, becoming 

 lengthened outward, constitutes what is 

 known as a cervical rib. This separate 

 ossific centre for the costal process has 

 also been found in the fourth, fifth, and 

 sixth cervical vertebrae. 



Lumbar Vertebrae. The lumbar 

 vertebrae (Fig. 33) have two additional 

 centres (besides those peculiar to the 

 vertebrae generally) for the mammillary 

 tubercles, which project from the back 

 part of the superior articular processes. 

 The transverse process of the first 



for tubercles on superior articular process. lumbar is sometimes developed as a 



FIG. 33. Lumbar vertebra. separate piece, which may remain per- 



1 for each trans- 

 verse process, 

 16 years. 



(sometimes 1) for spinous process (16 years). 

 FIG. 30 



By 3 centres. 



1 for anterior arch (1st year), 



not constant. 

 ~ 1 for each 

 lateral mass 



FIG. 31. Atlas. 



1 



before birth. 



By 7 centres. 



Zd year. 



6th month. 



1 for each lateral mass. 



1 for body (4th month). 

 1 for under surface of 

 body. 



FIG. 32. Axis. 

 2 additional centres. 



