OSSIFICATION OF THE VERTEBRA. i7 



The Spinal System. — ^The protovertebrse consist at first 

 of mere indifferent tissue ; and it is by a process of histologi- 

 cal differentiation within the protovertebral masses that, from 

 its deeper parts, one of the spinal ganglia and a cartilaginous 

 vertebral centrum — from its superficial layer, a segment of the 

 dorsal muscles, are produced. 



Chondrification extends upward into the walls of the dorsal 

 tube, to produce the neural arch and spine of each vertebra ; 

 and, outward, into the wall of the thoracic and abdominal part 

 of the ventral tube, to give rise to the transverse processes and 

 ribs. In fishes, the latter remain distinct and separate from 

 one another, at their distal ends ; but, in most reptiles, in 

 birds, and in mammals, the ends of some of the anterior ribs, 

 on both sides, unite together, and then the united parts coa- 

 lesce in the middle line to form a median subthoracic cartilage 

 ■ — the sternum. 



When ossification sets in, the centra of the vertebra are 

 usually ossified, in great measure, from ringlike deposits which 

 closely invest the notochord ; the arches, from two lateral de- 

 posits, which may extend more or less into the centrum. The 

 vertebral and the sternal portions of a rib may each have a 

 separate ossific centre, and become distinct bones ; or the 

 sternal parts may remain always cartilaginous. The sternum 

 itself is variously ossified. 



Between the completely-ossified condition of the vertebral 

 column and its earliest state, there are a multitude of grada- 

 tions, most of which are more or less completely realized in 

 the adult condition of certain vertebrated animals. The verte- 

 bral column may be represented by nothing but a notochord 

 with a structureless, or more or less fibrous, or cartilaginous 

 sheath, -nath or without rudiments of cartilaginous arches and 

 ribs. Or there may be bony rings, or enslieathing ossifications, 

 in its walls ; or it may have ossified neural arches and ribs 

 only, without cartilaginous or osseous centra. The vertebra3 

 may be completely ossified, with very deeply biconcave bodies, 

 the notochord remaining persistent in the doubly-conical inter- 

 vertebral substance ; or, ossification may extend, so as to ren- 

 der the centrum concave on one surface and convex on the 

 other, or even convex at each end. 



Vertebra which have centra concave at each end have 

 been conveniently termed amphicoelus ; those with a cavity 

 in front and a convexity behind, procodus ; where the position 

 of the concavity and convexity is reversed, they are opistho- 

 codous. 



