334 



DEVELOPMENT OF THE HUMAN BRAIN 



are developed other portions of the Cerehral Peduncles and also 

 the Thalami which rest upon and grow as ganglionic thickenings 

 from the latter structures. Its diminished cavity persists as the 

 future Third Ventricle {a). Its roof becomes gradually thinned 



Fig. 122.— Diagrams illustrating the Progressive Changes that take place during 

 the Early Stages of the Development of the Brain. (Mivart.) 



1. Early condition of Brain when it consists of three hollow vesicles (a b c), the 

 cavity of which is continuous with the wide cavity {d) of the primitive Spinal 

 Cord (»(.). 



2. Here the first vesicle or Fore-brain has developed the Pineal Body ( pi) above, 

 and the Pituitary Body ( v t) below. The wall at the anterior end of the first vesicle 

 is the future ' lamina terminaUs ' (0- 



3. This figure shows the Cerebrum {cr) budding from the first vesicle, its anterior 

 part (o) being prolonged as the Olfactory Lobe ; the cavity of the Cerebrum (the 

 incii)icnt 'lateral ventricle') communicating with that of the Olfactory Lo])e in 

 front, and with that of the first Cerebral Vesicle behind (this latter cavity persisting 

 as tlie future ' third ventricle '). The latter communication takes place through the 

 'foramen of Monro.' The walls of the three prinaitive vesicles are bei-oming of 

 unequal thickness, and the cavity (6) of the middle vesicle is becomiug reduced in 

 relative size. 



4. The Cerebrum is here enlarged, and the inequality iu thickness of the wall of 

 the primitive vesicles is increased. The greater development of the Cerebellum (<; /<), 

 the Pons (?)) and the Corpora Quadrigemina (7), show this distinctly. 



5. This figure shows the Cerebrum still more enlarged, and containing a tri-radiate 

 cavity {I, 1, 2, 3). The part destined to form the F.-rnix (./), which in No. 4 was 

 above, has now come to look slightly downwards, and prolongations from it begin to 

 extend towards the ' corpora albicantia' (<u a), v, corresponds with the situation ol 

 the ' velum interpositum.' 



