THE BONES OF THE TRUNK 55 



The skeletogenous sheath soon acquires great density at regular inte^^'als— 

 namely, opposite the intersegmental areas, which Ue between the muscle- 

 segments. Each of these dense parts is curved, somewhat hke a horse- 

 shoe, and is knovin as the Tertebral bow, or seleromere. The central part 

 of the bow lies on the ventral aspect of, or below, the notochord and is caUed 

 the hypoehordal bar. The lateral extremities of the bow divide each mto 

 wo Umbs dorsal and ventral. The dorsal iimbs. called the neural pre cesses, 

 right and left, extend dorsalwards. one on either side of the neural tube, on 



Nennl Tube 



Neural Proces' 



Costal Process ' ' ^^Sj^^S^f- -^ Vertebral Bow 



Chordal Sheath ■' ] Notochoid 



Hypocbordal Bar 



Fig. 38. — ^Thb Vertebral Bow (Schematic). 



the dorsal aspect of which they meet. They form the primitive neural arehes 

 of the vertebrae. The vetilral limbs, called the eost&l processes, extend ven- 

 tralwards on either side. In the thoracic region they form the primitive ribs; 

 in the cervical region, the primitive costal processes of the cer%ical vertebrae; 

 in the lumbar region, the primitive transverse processes (costal elements) of 

 the lumbar vertebrae ; and in the sacral region tiiey form the primitive costal 

 elements of the upper three sacral vertebrae. 



The vertebral bows £ire ztrranged in regular series, opposite the interseg- 

 mental areas, each ha\-ing on either side a neural process passing dorsal- 

 wards, and a costal process extending ventralwards. The neural processes, 

 right and left, of a vertebral bow, as just stated, meet on the dorsal asjject 

 of the neural tube, and they form the primitive neural arch of a vertebra 

 The costal processes, as just stated, give rise to the following parts : 



Thoracic Region : Lumbar Region : 



The Bibs. Transverse Processes (Costal 



Elements). 



Cervical Region : Sacral Region : 



The Costal Processes. Costal Elements of Upper Three 



Sacral Vertebrae. 



The hypoehordal brace or bar (central part of the vertebral bow) dis- 

 appears, except in the atlas, of which it forms the anterior arch. 



Cartilaginous Stage.— The skeletogenous sheath undergoes chondrification 

 at regular intervals opposite the intersegmental areas and the portions of the 

 muscle-segments which limit these areas, or, it may be. entirely opposite the 

 muscle-segments. In each part two nodules of cartilage appear, one on each 

 side of the notochord. These rapidly surround the chord, and so form the 

 cartilaginous body of a vertebra. The neural processes of the lateral extremi- 

 ties of a vertebral bow, which lie on either side of the neural tube and coalesce 

 along its dorsal aspect, also undergo chondrification jmd form the cartilaginous 

 neural arch of a vertebra, as well as the processes connected with that arch. 

 In this manner a cartilaginous sheath is constructed for the neural tube, 

 which tube to a large extent becomes converted into the spinal cord. 



