NERVOUS SYSTEMS OF BACKBONED ANIMALS 



21 



Amphibians, Reptiles, Birds, 

 unlike that comparison seems 

 dispelled by a study of de 

 velopment, which is the key 

 to the whole matter. What 

 takes place is broadly as fol- 

 lows (fig. 1025). The front 

 end of the nerve-tube grows 

 rapidly, and divides into three 

 successive swellings, which 

 we may term the Fore-, Mid-, 

 and Hind - Brains. These 

 three original swellings are 

 converted into the central 

 part or axis of the adult 

 brain, the front part of which 

 is called the 'Twixt- Brain, and 

 the hind part the Medulla 

 Oblongata or Spinal Bulb 

 (continuous behind with the 

 Spinal Cord), while the roof 



C.H. 



O.L. 



and Mammals look so extremely 

 hopeless, but such an idea is soon 



F. M. H. 



F. 



Pn. 

 f. C.H. T.B.\ O.L. 



I i ! 



Cb. 



Fig. 1025. Development of Vertebrate Brain, as seen in 

 longitudinal section, diagrammatic 



A, Brain and spinal cord at early stage; B, brain at later 

 stage enclosed in brain-case, the floor of which is shaded with 

 oblique lines; F., M., H., Sp.C. t fore-, mid-, and hind-brains, 

 and spinal cord; T.B., 'twixt-brain; C.H., cerebral hemispheres; 

 Olf., olfactory lobe projecting into nasal capsule; Pn. and Py., 

 pineal and pituitary bodies; O.L., optic lobes; M.O., medulla 

 oblongata; Cb. , cerebellum; N. N., notochord; n, optic nerve. 



of the middle section is thickened 



Cb. 



N. 



M.O. 



Fig. 1026. Brains of Trout (A), Frog (B), and Dog (c), seen from above, and drawn same length 



Olf., Olfactory lobes; C.H., cerebral hemispheres (Cl. is a cleft between them in B); T., 'twixt-brain; Pn., pineal 

 body; O.L., optic lobes; Cb., cerebellum; M.O., medulla oblongata; N., N., N., cranial nerves; n., n., spinal nerves. 



into a pair of swellings known as Optic Lobes, each of which, 

 in Mammals only, is divided into two smaller projections by a 



