and bulkier on the posterior vertebrae of this series. The 

 transverse process has a distinct posterodorsal muscular 

 process, which, on the lumbar vertebrae, becomes separate, 

 lying ventrolateral to the postzygapophysis. 



In their development, the thoracic vertebrae arise from 

 five ossification centers: bilateral neural arches, vifhich bear 

 the zygapophyses, transverse processes, and the articular 

 facets for the two heads of each rib; and the centrum with 

 its two epiphyses. 



The lumbar vertebrae have an anteriorly directed neural 

 spine and large pre- and postzygapophyses. The transverse 

 process is lacking, but its posterior process is well developed. 

 In its development, this type of vertebra is like the thoracic. 



There are three sacral vertebrae in the cat; the most an- 

 terior one is the main support for the pelvic girdle. The 

 transverse process of this anterior sacral is very thickened 

 and distally spread out to suture with the ilium. The sec- 

 ond sacral has a thick transverse process that joins in sup- 

 porting the ilium. The third sacral is not involved. In their 

 development, the sacrals are like the thoracics, except the 

 transverse process of the first sacral includes a secondary 

 center, a costal or rib element, arising below it, and forming 

 much of the articulation with the ilium. 



The neural canal continues posteriorly through the first 

 seven caudals and then is lost. The pre- and postzyga- 

 pophyses are present, as are anterior and posterior extensions 

 of the transverse process. The fourth to ninth caudals have 

 small "hemal arches" enclosing the caudal artery and vein. 

 These arches are directed forward and serve for muscular 

 attachment. Further posteriorly, the hemal (or haemal) arch 

 is represented by bilateral splints. 



Embryological development The terminology of the parts 

 of the developing vertebra is quite confusing, because it is 

 based on that of Gadow, who originally applied it to fish 

 vertebrae. Extrapolation from fish to mammal has not been 

 at all successful. 



In the course of the development of the mesoderm, somites 

 are formed (Figure 6-2). These are segmental blocks (readily 

 observed in whole mounts of embryo chicks) lying to 

 either side of the neural tube. These blocks develop a small 

 central cavity, the myocoel, which is a fissure separating the 

 somite into an outer dermatome and an inner myosclero- 

 tome, or myotome. From the ventromedial aspect of the 

 myotome, cells move downward and medially toward the 

 notochord. From these cells the sclerotomes of the more 

 advanced embryo form. 



The sclerotomes also show a segmental arrangement; the 

 cells are densest near the myotome and the middle of the 

 segment and more diffuse toward the notochord and the 

 intersegmental fissures. Observed in frontal section (as in 

 the 10-mm pig), these mesenchymatous masses are sepa- 

 rated by intersegmental fissures, or intersclerotomic fissures, 

 in which the segmental arteries lie medial to the larger seg- 

 mental veins. These blood vessels extend upward, from the 

 parent vessels, to either side of the notochord. 



Somewhat later, the intrasegmental or sclerotomic fis- 

 sures appear, marking the division of the sclerotomes into 

 anterior or cranial, and posterior or caudal halves, the 

 sclerotomites. These fissures are by no means complete nor 

 conspicuous. In the 10-mm pig, they are apparent, as is also 

 the trend for the caudal part of each sclerotome to appear 

 more dense than the anterior or cranial sclerotomite. The 



notochord 



sclerotome 

 myotome 

 myocoel 

 dermatome 



segmental nerve 



segmental vein 



segmental artery 



myotome 



narrow band of perichordal tube 

 matome 



ectoderm 



cranial sclerotomite 



—Caudal sclerotomite 



sclerotomic fissure 



vertebral 

 condensation 



B 



Figure 6-2. Three early stages in the blostemotic origin of the vertebrae in the mammal. (After 

 Patton, 1958) 



THE VERTEBRAL COLUMN • 143 



