2i8 ELEMENTS OF BIOLOGY 



properly, the prosencephalon, mesencephalon, and rhombenceph- 

 alon. Subsequently these divisions are further modified (Fig. 147). 

 The prosencephalon forms anteriorly and dorsally the telen- 

 cephalon from which the cerebrum of the adult arises, and the 

 DiENCEPHALON, which is the origin of the retina of the eye and of 

 the OPTIC NERVE, of a portion of the pituitary gland, and other 

 structures. The mesencephalon does not divide; it gives rise to the 

 OPTIC LOBES of the adult brain and to various tracts and paths of 

 communication between the fore- and hind-brains. The rhomben- 

 cephalon is the origin of two regions, the metencephalon, from 

 which the adult cerebellum arises, and the myelencephalon, which 

 becomes the medulla oblongata of the adult. These five portions 

 vary as to their relative size and degree of development in various 

 vertebrates, but all adult brains are derived from these basic parts. 

 The relative development of the parts in the brains of the five 

 classes of vertebrates is shown in the accompanying figure 

 (Fig. 148). 



The remainder of the neural tube posterior to the myelencephalon 

 becomes the spinal cord of the adult. In the wall and floor of the 

 primitive tube the cells increase in number; the outside diameter 

 of the tube increases and at the same time the walls encroach upon 

 the opening of the tube so that the inside diameter decreases. The 

 tubular character of the spinal cord is never completely obliterated, 

 however; a section through the adult human spinal cord shows a 

 small central canal still present. 



The potential nerve cells within the wall of the tube develop 

 axones and dendrites that traverse the cord anteriorly and posteri- 

 orly to establish :he nerve tracts that characterize its adult anatomy. 

 Some cells in the ventral region of the cord send out axones that 

 extend to the body muscles and constitute the motor elements of 

 the peripheral nervous system. 



The Sympathetic Nervous System (Fig. 146). For con- 

 venience here the sympathetic system is next to be considered. Dur- 



