284 



THE MUSCLES 



found in lower Vertebrata; and the two bellies may sometimes be fairly regarded 

 as distinct muscles, e.g. those of the digastric muscle. In some cases the central 

 tendon may be of some service in preventing the pressure upon subjacent tissues 

 which would occur if the muscle were at this point fleshy and free to thicken as it 

 contracted. For example, it has been suggested that the central tendon of the 

 omo-hyoid prevents that muscle from compressing the great vessels of the neck as 

 it crosses them beneath the sterno-mastoid. In one muscle, the rectus abdominis, 

 several of tlicse tendinous intersections are found. 



5. Nerve-supply. — This is of much importance, not only from a medical and 

 surgical point of view, when the paralysis or sjiasmodic contraction of individual 

 muscles has to be accounted for, but also because of the light which it throws 



Fig. 259. — Diagram showing Penniform Muscle, the Peroneus Longus ; and 

 bipexniform, the tibialis posticus. 



Peroneus longus 



Tibialis posticus 



Upon tlic actions of muscles, and the assistance wliich it sometimes gives us in 

 grouping thiMu. 



6. Action. — Most muscles act upon the levers formed ))y the movable bones 

 and cartilages. It will be convenient, however, first to speak of those which do 

 not act in this manner. When a muscle passes from a fixed point like a bone to a 

 freely movable point such as the under surface of the skin or mucous membrane, 

 it will simply tend to ap]>roxiniate the movable ])oint to the fixed origin. Such 

 muscles are found in abundance in the face, and the azygos uvula^ is a good 

 example of one Avhich acts upon mucous membrane. Again, the fibres of many 

 muscles run in parallel curves, which combine to form a curved band or sheet. 

 The first (>ffect of the contraction of such a muscle will be to straighten or flatten 

 out the curve. If this lies over a convex surface, as is usually the case, the action 



