42 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



three kinds of movements in the animal body, viz., vol- 

 untary, semivoluntary, and reflex. The cerebrum pre- 

 sides over the first — viz., the distinctly and consciously 



voluntary actions — such as 

 movements undertaken for 

 the first time and requiring 

 full attention and distinct ef- 

 fort. On the other hand, the 

 medulla and spinal cord pre- 

 side over the purely auto- 

 matic or reflex movements 

 — movements wholly with- 

 drawn from consciousness 

 and will, like those of respi- 

 ration and of the heart. The 

 thalamus seems to preside 

 over intermediate move- 



FiG. 28. — Illustrating function of 

 thalamus : s, sensory fiber ; m, 

 motor fiber ; ths, sensory cell of 

 thalamus ; csm, motor cell of 

 corpus striatum ; crs and crm 



ments — i.e., semiautomatic or 



sensory and motor cells of cere- , ,. t li-i .1 c . 1 



brum. habitual, like those of stand- 



ing, walking, flying, writing, 

 speaking, playing on a musical instrument, etc. All 

 these are acquired with some difficulty, the cerebrum 

 presiding, but gradually become easier and easier until 

 they require only the most general superintendence of 

 consciousness and will. If anything goes wrong, the 

 cerebrum takes control for a while and sets things right, 

 and again the movements lapse into semiautomatism. 

 It is as if the cerebrum gradually taught these under- 

 agents or employees — the thalamus and corpus striatum 

 — to do the work themselves, but under general super- 

 vision. To compare to an electric apparatus, it is as if 

 the sense impulse goes up to the cerebrum through a 

 sensory cell of the thalamus and comes back from the 

 cerebrum through a motor cell of the corpus to the 

 muscle, but a part of the current short circuits from the 



