CHAP, i.] THE NERVOUS SYSTEM IN THE ZOOLOGICAL SERIES. 371 



If we follow step by step the embryological evolution of any 

 superior vertebrate the transition is more graduated still : for 

 we see the nervous centres begin with the acranian form, then 

 assume by degrees the complete cerebral type, passing through 

 intermediate forms very analogous to those which persist in the 

 fishes and in the reptiles. 



In these two last classes the encephalic 

 nervous centres are represented by a 

 series of vesiculiform expansions (Figs. 

 60 and 61). In like fashion the ence- 

 phalon of the superior vertebrates is 

 composed first of all of five vesicles, 

 which have been called, going successively 

 from the front to the back, anterior 

 brain, intermediary brain, median brain, 

 posterior brain, and terminal brain 

 (Figs. 62, 63, 64, 65). With the inferior 

 face of the anterior brain are connected 

 the olfactory bulbs, which furnish the 

 nerves of smell. 1 



The anterior brain is divided by an 

 antero-posterior fissure into two hemi- 

 spheres, which, continuing to increase in 

 size, become the principal cerebral mass, 



, ., , Brain and marrow of the frog : 



the cerebral hemispheres, and cover tne A, upper aspect ; B, nether 



, . , , . . , , aspect ; a, olfactory bulbs ; &, 



Other encephalic expansions, tne more anterior brain; c, median 

 ,, , , . IT brain; d, posterior brain; e, 



the vertebrate is intelligent. 



elongated marrow ; i, infundi- 

 bulum; s, rhomboidal fosse; 

 m, spinal marrow; t, its ter- 

 minal thread. 



The intermediary brain divides also 

 into two masses, which, as we shall see 

 ere long, play a very important part in the cerebral functionment. 

 These are the optical expansions or optical layers. 



The median brain reduces itself more and more, and is no 

 longer represented in the human brain by anything but four 

 small tubercles, called quadrigeminous tubercles. 

 1 Gegenbaur, loc. tit. pp. 681-690. 



B B 2 



