502 Animal Biology 



tain synapses a much simpler type of impulse transmission may take place 

 than what has been described. The rate of impulse transmission through 

 a synapse is slower than along a nerve under ordinary conditions. In 

 case the tip of an axon is adjacent to a gland or muscle, the neurohumor 

 stimulates and causes a secretion, or a movement, accordingly. In the 

 case of impulse transmission from nerve to muscle the chemical is spe- 

 cifically known as acetylcholine. The chemical known as synipathin be- 

 tween a nerve and the heart results in speeding up the latter. The ex- 

 planation for a lack of continuous impulse transmissions through a 

 synapse is based on the presence of an enzyme called cholinesterase which 

 oxidizes (destroys) the acetylcholine, thus preventing a constant flow of 

 impulses through a synapse. Impulses travel through when acetylcholine 

 is present but do not when the latter has been destroyed by the cholin- 

 esterase. The natural resistance offered by synapses may be modified 

 by nerve impulses. In some cases one impulse strengthens another and 

 is known as reinforcement, while in other instances one impulse may 

 cancel the effect of another which is called inhibition. Impulses may 

 cross a synapse if reinforced by others or may not cross if they are in- 

 hibited. This complex reinforcement-inhibition relationship may explain 

 many of the phenomena of the nervous system. 



The human hrain (Figs. 246 and 247) consists of (1) cerebrum, (2) 

 cerebellum, (3) midbrain, (4) medulla oblongata, and (5) pons varolii. 



Central 

 nervous 

 system 



Autonomic 

 nervous 

 system 



Summary of the Human Nervous System 



'Cerebrum, which is large, ovoidal, convoluted, and made of 



two hemispheres with five lobes 

 Cerebellum, which is smaller, oval, nonconvoluted but with 

 smaller furrows (sulci) 



Midbrain, which is short and connects the cerebellum with 

 Brain ■{ the pons varolii 



Medulla oblongata, which is pyramid shaped and continues 

 ^ with the spinal cord 



Pons varolii, which is in front of the cerebellum between the 



midbrain and medulla oblongata and which connects the 



parts of the brain (Fig. 247) 



Cranial nerves (12 pairs) and their end organs (Fig. 247) 



Spinal cord for reflexes and pathways to and from the higher nervous 



centers (Fig. 247) 



^Spinal nerves (31 pairs) and their end organs 



Sympathetic, which has centers, ganglia, and plexuses in the cervical, 

 thoracic, and lumbar regions of the spinal cord 



Parasympathetic, which consists of the centers and ganglia of the 

 cranial and sacral parts of the autonomic system 



Enteric, which consists of the part of the autonomic system associated 

 with the walls of the alimentary tract 



