BRAIN. 



435 



When three toes only are present the hallux is suppressed. 

 In the ostrich digit No. 2 is also absent. In the penguins the 

 fibula has the same length as the tibia, and the metatarsals 

 though fused, are more distinct than in 

 other birds. 



In the development of the foot there are 

 said to be at first three proximal tarsals and 

 five distal. The proximal elements unite 

 while still in the cartilaginous condition into 

 one piece which then ossifies and fuses \Arith 

 the tibia. The five distal elements are also 

 said to unite to one cartilage which ossifies 

 and fuses with the second, third, and fourth 

 metatarsals. The latter are at first separate, 

 but later fuse. Wlien the compound meta- 

 tarsal so formed elongates, as it generally 

 does, the first metatarsal does not share lq 

 the elongation but remains distinct at the 

 distal end ; in one or two cases (e.g. Phaethon) 

 it fuses with the lower end of the tarsometa- 

 tarsus. A centrale (or sometimes two) is 

 said to be sometimes detectable in the embryo 

 and even to persist in the adult as a distinct 

 bone on the posterior surface of the joint. 



The brain of birds (Fig. 240) is much 

 more highly developed than that of 

 reptiles, and completely fills the roomy 

 cranial cavity. The hemispheres are, 

 indeed, still without superficial con- 

 volutions. They cover not only the 

 thalamencephalon, but also the two 

 large, laterally displaced corpora bige- 

 mina.' The differentiation of the cere- 

 bellum is still further advanced, since 

 there is a median part corresponding 

 to the so-called vermis of Mammalia 

 and marked by transversely directed 

 sulci and small lateral lobes. 



In consequence of the cervical 

 flexure of the embryo the medulla 



oblongata forms an angle with the spinal cord, the posterior 

 columns of which diverge from one another in the posterior 



Fio, 



240. — Brain and spinal 

 cord o£ a pigeon. C cere- 

 bellum ; Cb optic lobes ; E 

 cerebral hemispheres ; Mo 

 medulla oblongata ; Sp spinal 

 nerves. 



