VI.] THE COMMON FROG. 73 



a continuous ossification of the sheath of the noto- 

 chord, and never consists of distinct vertebra. Never- 

 theless, the small bony arches which are at first 

 distinct coalesce with it. These arches are called 

 " neural " because they arch over the hinder part of 

 the spinal marrow. The great nerve of the leg (the 

 sciatic nerve) proceeds outwards on each side through 

 a foramen situated at the anterior end of the coccyx 

 from the spinal marrow — the spinal marrow being 

 that structure which gives origin to the great mass of 

 the nerves pervading the entire frame (fig. 32). 



The skull of the frog presents numerous points of 

 interest, but only four of these can be here referred 

 to, as other matters demand our attention. 



The first of these four relates to its mode of 

 articulation with the vertebral column. As has been 

 said, the first vertebra presents a pair of concavities to 

 the skull. The skull develops from its hinder (or 

 occipital) region a corresponding pair of articular 

 convexities or " condyles." Now in this matter the 

 frog differs from both birds and reptiles, every 

 member of those classes possessing a single median 

 (occipital) condyle for articulation with the vertebral 

 column. 



Yet every member of the frog class, not only every 

 toad and newt, but also every species of the Ophio- 

 morpha, and even every one of the long extinct 

 Labyrinthodons (with the doubtful exception of the 

 probably immature and larval Archegosaitj^iis) has a 

 similar pair of occipital condyles. The interesting 

 matter is that man and all beasts have also two 

 occipital condyles. Is this then a mark of affinity, 



