4i8 CHORD AT A. 



the second and third form the thyroid^ and probably the 

 arytenoid cartilages of the larynx. i 



The Vertebra. — A typical vertebra consists of a] 

 centrum or main axis, above which is a bony neural arch " 

 covering in the spinal cord. It is often surmounted by 

 a more or less prominent median neural spine. From each 

 side of the neural arch there usually protrudes a lateral 

 process known as the transverse process. In the anterior 

 part of the vertebral column the vertebra usually bears a 

 rib, which is articulated to the centrum by its head or 

 capitulum and to the transverse process by its tuberculuni. 

 The rib may, however, become completely fused on to the 

 vertebra (cervical), or it may be attached only to the 

 transverse process, or may become fused with the transverse 

 process (lumbar and sacral). In the region behind the 

 sacrum there is often a haemal arch, but in mammals this is 

 only found in a few types in the form of chevron bones 

 which articulate between the vertebrae. In Mammalia the 

 centra have epiphyses or caps of bone, and these are usually 

 flat, though they may be opisthoccelous in some of the 

 cervicals. At the front and hind -end are anterior and 

 posterior zygapophyses which serve as articulations between 

 the vertebrae. 



The vertebrae are usually divided into — (i) cervical, 

 (2) thoracic, (3) lumbar, (4) sacral and (5) caudal. The 

 cervicals are defined as lying between the skull and the 

 first thoracic, or the first vertebra that has a pair of ribs 

 which meet the sternum. The thoracic vertebrae bear ribs 

 which meet the sternum. In all the higher Vertebrata the 

 sternum is formed from the fusion of the distal ends of the 

 ribs. 



Development, — The embryo has a notochord, around which is 

 formed a continuous mesoblastic membranous or skeletogenous sheath. 

 This sheath extends dorsally round the neural tube {cf. cranium). 

 Paired masses of cartilage then appear above and below in the sheath. 

 Their bases fuse across from side to side and dorso-ventrally to form the 

 cartilaginous centrum, and the dorsal arches grow up round the spinal 

 cord to form the neural arch. Ossification then takes place, there 

 being usually several centres of ossification. 



Appendicular. — All Vertebrata above Cyclostomata 

 (and exceptions) have two pairs of limbs and limb-girdles, 

 an anterior or pectoral and a posterior or pelvic. 



