346 ZOOLOGY. 



fore brain); inf., infundibulum; med., medulla oblongata; mes., mesencephalon or mid 

 brain; met., metencephalon or hind brain; my., myencephalon or medulla; n 1 , olfactory 

 nerve; n 2 , optic nerve; ol., olfactory lobe; op., optic lobes; o.th., optic thalamus; pr., 

 prosencephalon, or fore brain; p.v., pons Varolii; s.c., spinal cord; th., thalamen- 

 cephalon or " between " brain. 



Questions on the figures. What portions of the adult brain are pro- 

 duced from each of the three primary lobes? Where are the principal out- 

 growths, thickenings and thin portions of the wall? In comparison with 

 figure D what portions of the brain become highly developed in the 

 higher Vertebrates? Make a diagram based on D, which will show the 

 general relation of these parts in man. Compare the diagrams with the 

 table on page 347, and verify the statements there. 



regions of the adult. The brain must be considered merely as 

 the specially modified anterior portion of the spinal cord. 



Three sets of changes occur in the development n" the adult 

 vertebrate brain from this primitive condition : 



1. The axis becomes more or less curver 1 , the concavity 

 being ventral. 



2. Outpocketings of the walls occur, in special regions, 

 whose cavities (ventricles) retain connection with the central 

 cavity (e. g., the hemispheres). See Fig. 170, h, pi. 



3. Thickenings or thinnings of the roof, sides, or floor of 

 the tube may produce lobes and affect the size of the cavity of 

 the tube. The accompanying diagrams and table will furnish 

 an outline from which the teacher may, if he desire, pursue 

 the details somewhat further. 



359. That portion of the central nervous system not en- 

 closed in the skull is called the spinal cord. It is surrounded 

 and protected by the dorsal arches of the vertebrae. The 

 cord is nearly circular in cross-section, is somewhat enlarged 

 in the regions of the appendages, tapers toward the posterior 

 end and is divided into symmetrical right and left lobes by a 

 dorsal and a ventral longitudinal groove (see Fig. 171, c//.). It 

 possesses a central canal continuous with the cavities of the 

 brain. The outer part of the cord (Fig. 171, w.) is composed 

 of the white matter (longitudinal nerve fibres) and the interior 

 portion, of gray matter (a mixture of nerve-cells and fibres). 

 This is somewhat the reverse of the condition found in the 

 brain. 



