Mr. Le Gros Clark on the Mechanism of Respiration. 129 



imitation of their action. In other words, the relation of the fibres of this 

 muscle to the ribs is such, that a passive imitation of the action ascribed to 

 them is to make tense and not to relax them ; they have to run over a 

 longer surface — a wider barrel. But the fact is, that the lower angle of the 

 scapula is neither fixed nor in any marked degree drawn up in deep inspi- 

 ration ; but the angle and vertebral costa are carried further away from 

 the spine ; and as the serratus is, as already remarked, not visibly or 

 tangibly in action, this result can be accomplished only by the costal 

 attachments of the muscle being removed to a greater distance from the 

 spine, by the expansion of the chest. Each digitation of the muscle acts 

 at an increasing disadvantage as we ascend from the last to the first ; 

 therefore, if the lowest digitation is incapable of acting as an elevator of 

 the ribs, the same negative conclusion must, d fortiori, be arrived at as 

 regards the rest of the muscle. 



It is no doubt true that the serratus is in action during deep inspira- 

 tion, when the arm is raised. But this contraction is persistent during 

 expiration also ; and the action in each instance is explained by the fact 

 that this muscle is required to assist the trapezius in rotating outward the 

 lower angle of the scapula, and maintaining it in that position, in order to 

 accommodate the relation of the glenoid cavity to the head of the humerus, 

 and to afford a fixed attachment for the action of the deltoid. The chest 

 is, under these circumstances, the relatively fixed origin of the serratus, 

 and the scapula its movable insertion on which it rests. 



18. A parity of reasoning applies to the pectoralis major muscle. This 

 muscle is described as assisting in forced inspiration, by raising the ribs 

 and dilating the chest. I think this assumption incorrect for the fol- 

 lowing reasons : — 



(1) A necessary condition to the action of this muscle on the thorax is 

 the relative immobility of the scapula and humerus ; but in a healthy 

 chest the most powerful inspiratory effort may be made without any such 

 assistance ; and the pectoral fibres may be felt and seen to remain during 

 this effort in a state of complete inaction. When the humerus is fixed, 

 the fibres are in action ; but this condition is due to their agency in fixing 

 the bone. 



(2) The clavicular and upper portion of the sternal fibres may be con- 

 sidered as incapable of raising the ribs, whilst the arm is not elevated ; 

 the lower fibres alone can be supposed to possess this property or power. 

 Now the lowest fasciculus is not attached to the ribs, but terminates in 

 the aponeurosis of the abdomen by a broad fibrous expansion : this portion 

 of the muscle can take no part in raising the ribs. The set of fibres above 

 these arises from the sixth rib and its cartilage, and from the adjacent 

 portion of the sternum : they are inserted into the upper part of the outer 

 margin of the bicipital groove, in a horizontal plane with the sternal attach- 

 ment of the third rib and the spinal attachment of the sixth rib. In deep 

 inspiration the effect of forward movement of the sternum will nearly, if 



