1 50 ZOOLOGY. 



femur is long, thin and has a double bend ; the tibia and 

 fibula are fused together ; the astragalus and calcaneum 

 are remarkably lengthened, united at either end, but quite 

 distinct between ; the calcaneum has no projecting heel. 

 These are followed by two other small tarsals. There are 

 five digits, each with a metatarsal and phalanges, the 

 formula being 22343. In addition there occurs on the 

 pre-axial side of the hallux what appears to be a rudimentary 

 extra digit the caJcar ; it consists of a metatarsal, with 

 occasionally a phalanx or two as well. 



12. Nervous System. The central and peripheral 

 nervous systems are built on exactly the same plan as in 

 the rabbit, but show a general simplification. 



The spinal cord has a general similarity to that of the 

 rabbit, but is very short, and the spinal nerves number 

 only ten pairs altogether, the first nine coming out behind 

 the corresponding vertebrae, and the tenth through the 

 apertures in the urostyle already mentioned. Thus, unlike 

 the rabbit, there is no nerve emerging between skull and 

 atlas. The first spinal nerve corresponds in distribution 

 with the rabbit's hypoglossal nerve, a point we shall refer to 

 again when we speak of the skull. The second and third 

 constitute the brachial plexus. The seventh, eighth, and 

 ninth form the sciatic plexus going to the hind limb. Evi- 

 dently we cannot regard these nerves as corresponding to 

 those of the rabbit, each to each in order from in front 

 backwards : if we did so we should be forced to the ridiculous 

 conclusion that the frog's hind-legs were homologous with 

 the rabbit's fore-legs, since both are supplied by similarly 

 numbered spinal nerves. We must recognize that we may 

 have two animals showing metameric segmentation, and yet 

 that the homologous regions may be divided into a very 

 different number of metameres in the two cases. 



13. The Brain (.figs. 75 and 76) shows all the same 

 essential parts as the rabbit's, and the ten pairs of cranial 

 nerves correspond exactly in origin, and very closely in 

 distribution, with the first ten of the rabbit. But the 

 general arrangement of parts is simpler; for the thalam- 



