102 PHYSIOLOGY CHAP. 



support a joint and keep its articular surfaces in contact, although 

 it is undeniable that this pressure is a considerable factor. 



Owing to their conformation the joints and the soft parts 

 which surround them (muscles, capsules, ligaments) not only 

 serve to connect the segments of the limbs, but also limit their 

 movements. Thus the olecranon of the ulna during the extension 

 of the forearm comes in contact with the dorsal surface of the 

 humerus, and prevents further extension. The same function 

 is exerted by the so - called ligaments of arrest ; the lateral 

 ligaments of the knee-joint, which run from the internal and 

 external condyles of the femur to the internal condyle of the 

 tibia and the head of the fibula, are stretched during the extension 

 of the leg, and limit this movement to 180. 



II. The discussion on the physiology of muscle in Chapter I. 

 refers particularly to bundles of parallel fibres of uniform 

 length, in which the total action represents the sum of the 

 actions of each fibre. But muscles with parallel fibres like 

 the sartorius and the frog's hypoglossus are rare ; the structure 

 of the muscle is usually less simple. In addition to long muscles 

 and short muscles, cylindrical, and spindle-shaped, and flat muscles, 

 anatomists distinguish fan-shaped muscles, semipennate muscles, 

 and pennate muscles, according to the direction of the fibres, the 

 form of the tendons, and the manner in which the muscle bundles 

 are inserted. 



In fan-shaped muscles the different parts may act separately 

 or all together. The deltoid is a classical example. This muscle 

 raises the arm forward, backward, or from the side, according 

 as only the front or back portion or the whole acts. In the 

 latter case the movement (as occurs when several forces act 

 simultaneously in different directions) follows the diagonal given 

 by the parallelogram of the forces, which causes the arm to be 

 raised in the lateral plane. 



Semipennate and pennate muscles are more common. In 

 these a tendon penetrates deep into the belly of the muscle, and the 

 muscular fibres run out from it obliquely in one or more directions. 

 In such muscles the line of junction of the points of attachment 

 does not coincide with the direction of the fibres, and when the 

 whole muscle contracts, the effect is the sum of the values, 

 calculated for each fibre separately. The gastrocnemius, the 

 biceps, and brachialis anticus, and the flexors for the arm, are 

 examples of pinnate muscles. 



Generally speaking, in muscles with parallel fibres, the 

 diameter and cross -section is proportional to their strength, 

 while their length is proportional to the range of the movements 

 they can produce. But in pennate muscles the strength and 

 range of the movements cannot be deduced from their section 

 and apparent length : there are short muscles which appear to be 



