188 FOUNDATIONS OF BIOLOGY 



consists of these divisions, known as PROSENCEPHALON, 

 DIENCEPHALON, mid-brain or MESENCEPHALON, EPENCEPHA- 



LON Or CEREBELLUM, and METENCEPHALON Or MEDULLA 



OBLONGATA, the latter merging into the spinal cord. Usually, 

 however, the prosencephalon gives rise to a pair of 

 PARENCEPHALA, or CEREBRAL HEMISPHERES, which are des- 

 tined gradually to overshadow in development all the other 

 parts of the brain and to become the seat of consciousness 

 as well as of the higher mental life in general. 



Finally, the development from the prosencephalon, or 

 from the cerebral hemispheres when present, of a pair of 

 RHiNENCEPHALA,or OLFACTORY LOBES, completes the establish- 

 ment of the chief brain chambers. The further changes 

 which transform the more or less linear series of vesicles 

 into the increasingly complex and compact brain of higher 

 forms are due to bendings, or FLEXURES, and to unequal 

 thickenings and outgrowths of the chamber walls. For in- 

 stance, the upper and lower surfaces of the diencephalon give 

 rise to the PINEAL BODY and the INFUNDIBULUM respectively, 

 while from similar regions of the mesencephalon are de- 

 veloped the OPTIC LOBES and CRURA CEREBRI. Hand in hand 

 with these changes the primary cavities (VENTRICLES) of the 

 chambers undergo a gradual constriction, but throughout all 

 there persists at least a remnant of the original tubular cavity 

 which is continuous with that of the spinal cord. (Fig. 105.) 



The brain and spinal cord lie, as we know, imbedded in 

 the muscles forming the dorsal part of the body wall, and 

 are protected and isolated by a cartilaginous or bony tube 

 formed by the skull and neural arches of the vertebrae. 

 The sole paths of nervous communication between the 

 central system and the rest of the organism and its sur- 

 roundings are a series of pairs of CRANIAL and SPINAL NERVES. 

 These arise at fairly regular intervals from one end of 



