36 Anatomy of Skeleton 



There is occasionally a distinct costal element in the ossification of the first lumbar 

 transverse process, but this does not form the whole of the process, and the corre- 

 spondence of this with a dorsal rib is not quite certain : it is interesting to consider 

 this region in the lumbar series and compare it with the transverse processes of the 

 lower dorsal vertebrae. 



In Fig. 29 the last two dorsal and first lumbar vertebrae from a well-marked set of bones are 

 represented and the tubercles on the transverse region are shown and numbered ; at the same time 

 a scheme of a dissection to show the muscle-attachments is given, which includes the process of 

 the tenth dorsal vertebra. The mammillary tubercles (i, i) are plainly enlarged "muscular" 

 tubercles for Multifidus spinae, while the accessory tubercles (2, 2) evidently serve the same function 

 for the Longissimus dorsi ; the different relation to the articular processes only has to do with the 

 different positions of these processes in the dorsal and lumbar regions. But there is another tubercle 

 on the last two dorsal vertebras, numbered 3 in the figure, and this appears to be in series with the 

 costal support of the tenth transverse process and, like it, to give origin to Levator costae fibres in 

 each segment. In these two vertebrae, therefore, we may perhaps look on the third tubercle as in 

 some measure comparable with the others, being accentuated as a point of muscular attachment, 

 or we may consider it a representative of the " costal tubercle " or rib-carrying process ; probably 

 its value might be expressed by describing it as a costal tubercle specialised as a muscular process. 

 But in the first lumbar transverse process there is great difficulty in recognising this third tubercle . 

 Is it elongated to form the " transverse process," or is it lost in the base of this process, which is a 

 true rib in value and supported on the third tubercle fused with it ? The first of these alternatives 

 is suggested to the eye by the schematic figure and the second by the conditions seen in the larger 

 drawing, but in each case, the opinion would only be founded on appearances. Examination of a 

 first lumbar gives one the decided impression that the proximal parts of a rib are fused with the 

 bone, and the rare occurrence of a costo-transverse foramen here supports the view. Lumbar ribs 

 articulating with the body also support it, but other ribs occur that are only at the ends of the 

 transverse processes and these seem to favour the first of the alternatives. Possibly the extra 

 costal centre in the transverse process represents a fused head end of a rib and the occasional 

 lumbar rib at the extremity may represent the shaft belonging to it. 



Ossification of Vertebrae. The vertebrae are preformed in cartilage, the carti- 

 laginous structure showing a centrum and two separate halves of a neural arch, with 

 the various processes represented at first only in concentrated mesenchyme into 

 which the chondrifying process extends later. 



Ossification begins during the seventh week by the formation of three primary 

 centres, one for the centrum and one for each half of the neural arch. The centre 

 for the centrum is probably double in origin, fusing rapidly into one. 



The primary centres appear at different times in different regions : thus those 

 for the arches appear first in the cervical region (Axis) and succeed one another from 

 above downwards, while the centrum ossifies earliest in the lower dorsal region and 

 " spreads " from thence in both directions. So the cervical arches can be said to ossify 

 before their centra, while the reverse is the case in the lumbar and lower dorsal 

 regions : there appears to be considerable individual variation in the rapidity of the 

 successive appearances of centres, but it may be said that by the end of the third 

 month there are primary centres in all the true vertebrae. 



At birth the corresponding primary bony parts are distinct, but joined by cartilage. 

 The neural halves join dorsally during the first year or so, fusion commencing in the 

 lumbar region, and a few years later the neural arches effect junction with the bodies, 

 the process commencing in the dorsal vertebrae. 



After puberty secondary centres appear and fuse with the primary bone by the 

 age of twenty-one. These centres occur in the cartilage covering the upper and lower 

 surfaces of the body (epiphysial plates) and the tips of the transverse and spinous 



