264 



THE HUMAN MECHANISM 



1. Fundamental structure of the nervous system; the brain 

 of a frog. The human spinal cord and brain are so com- 



plicated that it is best to 



Forebrain 



' Tweenbrain 

 Midbrain 



Hindbrain 



Spinal 

 Cord 



EIG. 105. The brain and spinal cord of the frog 



On the left is a longitudinal, right-to-left section, 

 showing the central canal and the ventricles of the 

 brain ; on the right the dorsal view of the brain and 

 cord. A, the c'erebral hemispheres ; B, the optic lobes ; 

 C, the cerebellum; D, the bulb; E, the spinal cord 



study first the nervous system 

 of a simple verte- 

 brate like the frog, 

 for the fundamen- 

 tal plan of struc- 

 ture is the same in 

 both. The spinal 

 cord is a relatively 

 thick-walled tube, 

 I p the walls of which 



J I I are composed of 



1 white and gray 



matter, the minute 

 bore, or lumen, of 

 the tube being 

 known as the cen- 

 tral canal. The ar- 

 rangement in the 

 brain is similar, but 

 here the central 

 space is no longer 

 a small tube of even 

 bore, but consists 

 for the greater part 

 of irregular cavities 

 known as the ven- 

 tricles of the brain, 

 while the walls 

 consist of masses 

 of gray and white 



matter varying in size, shape, and relation to each other. 



Fig. 105 will assist the student in understanding this plan 

 of structure. Anteriorly the spinal cord is continued in the 



