THE FROG 23 



larger or smaller, of it are generally to be found. A word of 

 warning is here necessary. Since the vertebral column is 

 sometimes called the spinal column, or simply the spine, and 

 since the column of nervous tissue which is enclosed by the 

 arches of the vertebral column is generally called the spinal 

 cord, the beginner is apt to confuse the notochord with the 

 spinal cord. It must be clearly understood that the nervous 

 structure, the spinal cord, lies above and not inside the 

 centrum, and that it is the notochord, the embryonic structure, 

 which occupies the cavity when present in the vertebral 

 centrum. 



The arch of the vertebra (fourth orfifth) consistsof aflatdorsal 

 expansion, united on either side to the centrum by a narrow 

 pedicle. When the vertebrae are jointed together in the natural 

 position, spaces are left between the pedicles of successive 

 vertebral arches through which, in life, the spinal nerves 

 emerge from the neural canal. From each side of every arch 

 a stoutish bony piece, the transverse process, projects 

 outwards. 



Each arch is yoked to its fellows in front and behind by a 

 pair of articular processes, already referred to as zygapophyses. 

 These processes project forward and backward from the outer 

 angles of the arch. Their articular surfaces are flat, and it is 

 an invariable rule in the Vertebrata that the articular faces of 

 the anterior zygapophyses look upward or inward, those of 

 the posterior zygapophyses downward or outward. Thus one 

 can tell the anterior end of a single vertebra by mere in- 

 spection of the zygapophyses. In addition to the processes 

 already detailed, each arch has a ridge running along its 

 median dorsal line. This is called the neural spine, and it is 

 much smaller in the frog than in many other Vertebrata. 

 The transverse processes do not all stand at right angles to the 

 axis of the vertebral column, but point in various directions. 

 There is no transverse process in the first vertebra ; those of 

 the second are directed downward and forward; those of the 

 third very slightly backward; those of the fourth much more 

 backward ; those of the fifth, sixth, seventh, and eighth stand 

 nearly straight out; and those of the ninth are very large and 

 stout and point very markedly backward. Each transverse 

 process has a little tip of cartilage called an epiphysis. 



The first or atlas vertebra differs markedly from the rest. 



