The Hidden Machinery
A receptor receives energy from outside the body and thus initiates the nerve impulse which traverses the nerve path until it finally excites the muscles and glands. The glands are either duct glands or ductless glands. One of the greatest sources of constant activity to be found in the hidden machinery arises in the muscles, both smooth and striped. The striped muscles are bundles of thread-like cells that run lengthwise throughout the muscle. The muscle fibre is the structural element of the muscle just as the neuron is the structural element of the nervous system. The muscle fibres of the striped muscles are excited directly by the motor nerves. The axones of the motor nerves lose their medullary sheaths upon reaching the muscles, and break up into many collaterals.