Structure and function are reviewed in the masticatory muscles and in the 
muscles of the lower face and tongue. The enormous strength of jaw closure is in 
large part due to the pinnated arrangement of the muscle fibres in the masseter. 
This muscle, like other masticatory muscles, is unusual in that the cell bodies 
of the muscle spindle afferents lie in the brain stem rather than in an external 
ganglion; spindles are absent in the lower facial muscles. Although few data are 
available, the numbers of motor units in the masticatory muscles, and probably 
in the lower facial muscles also, appear to be much greater than in limb 
muscles. The motor units in the facial and tongue muscles are largely composed 
of histochemical type II ('fast-twitch') fibres, but in the masticatory muscles 
there are substantial numbers of fibres intermediate between type I ('slow 
twitch') and type II, and fibre type grouping is present. In comparison with 
limb muscles, there is little information on ageing changes in oro-facial 
muscles. The masticatory muscles do, however, show some atrophy and loss of 
X-ray density, while motor unit twitches are prolonged. Strength is reduced in 
the tongue and masticatory muscles. It is known that limb muscle properties are 
largely governed by their innervation, both through the pattern and amount of 
impulse activity, and the delivery of trophic messengers; the situation for 
oro-facial muscles is unclear. The structural and functional differences between 
the two types of muscle indicate the need for conducting ageing studies on the 
oro-facial muscles, rather than relying on extrapolations from limb muscles.