406 TEXTBOOK OF ZOOLOGY 



posed of one motor and one sensory neurone; however, it is usually- 

 more complex. The classical example involves the spinal cord and 

 a spinal nerve. This is known as a reflex of the first level, because 

 it returns the motor impulse over the motor fibers of the same nerves 

 which brought in the sensory impulse. The motor axone carrying 

 the impulse from the motor nerve cell in the gray matter usually j 

 ends in a muscle cell or a gland. There is no protoplasmic union 

 between the axone of the sensory neurone and the dendrite of the 

 motor, for these come in contact only by a synapse which brings them 

 in close proximity. It has been found experimentally that nervous 

 impulses may be conducted in either direction by the fiber but can 

 cross a synapse only from axone to dendrite, thus serving like a valve 

 in a pipeline. Reflex actions may be in the form of motion, as with- 

 drawal from unexpected pain, or shivering or formation of goose 

 flesh, or the contraction of the pupil of the eye with increased light 

 intensity. Still other reflex actions include secretion by glands, 

 breathing, movements of speech, individual actions included in walk- 

 ing, and others. 



Functions of the Spinal Cord. — This organ serves as a system of 

 reflex centers which control the actions of glands of the trunk, vis- 

 ceral organs, and skeletal muscles. The spinal cord is also a nerv- 

 ous pathway between the brain and numerous organs of the body. 

 It is said that more than half a million neurones join the cord 

 through the dorsal roots of the spinal cord. 



Functions of the Divisions of the Brain. — Conscious sensations 

 and intelligence are centered in the gray matter or cortex of the 

 cerebrum. This section controls voluntary actions and provides 

 memory associations. The diencephalon serves as a center for spon- 

 taneous actions. The midbrain is one of the centers of coordinated 

 movement which has to do with posture and eye muscles. The cere- 

 bellum is another center of coordinated movement, particularly with 

 reference to equilibrium. The impulses from the muscles, tendons, 

 joints, and semicircular canals of the ear are coordinated so that in 

 a movement or posture the proper muscles may be contracted to the 

 proper extent at the proper time. Below and behind the cerebellum 

 is the medulla oblongata which controls breathing and may be an 

 inhibitor on heart action. It also regulates digestive secretions, 

 movements of digestive organs, and vasomotor activity of the blood 



