36 THE FROG CH. in 



3. The shoulder-girdle ox pectoral arch, a.n inverted arch 

 of bone and cartilage nearly encircling the anterior part of 

 the trunk and giving attachment to — 



4. The bones of the fore-limbs. 



5. The hip-girdle or pelvic arch, an apparatus shaped 

 somewhat like a bird's " merrythought " : it is attached in 

 front to the ninth vertebra and behind gives attachment to — 



6. The bones of the hind-limbs. 



The Vertebral Column. — The essential structure of a 

 vertebra may be best studied by examining any of the nine 

 from the second to the seventh : the first, eighth, and ninth 

 present certain peculiarities, and are best left till last. 



The whole vertebra (Fig. 8, B) has something the form of 

 a signet-ring with its sides produced into two outstanding 

 projections. The part corresponding with the stone of the 

 ring is ventral in position, and is called the body or centrum 

 {en), the form of which is proccelous, i.e., its anterior face is 

 concave, its posterior face convex, and both faces are 

 covered with a thin layer of cartilage. The part corre- 

 sponding with the circle of the ring is the neural arch 

 (pd, Im) : it arches over the spinal cord and is produced 

 in the middle line above into a blunt projection, the neurc^t 

 spine {n. sp). From the arch is given off, on each side, the 

 large outstanding projection already referred to, the 

 transverse process {tr. pr), which is tipped with cartilage in 

 the second, third, and fourth vertebrae. 



The neural arch gives off from its anterior face, just above 

 the origin of the transverse processes, a pair of small shelf- 

 like projections, the articular processes or zygapophyses {a. zyg). 

 Each has its upper surface flat and smooth, and covered with 

 a thin layer of cartilage. A similar pair of processes spring 

 from the posterior face of the arch, but have the smooth, 

 cartilage-covered surface looking downwards. 



