134 ARBOREAL MAN 



Charles Bell the " external " and the other the " in- 

 ternal " respiratory system. It is possible for these two 

 systems to be combined, and to act in consort. It is 

 easy to realize that, with the action of the external 

 respiratory system, a more effective inspiration will be 

 produced if the internal respiratory system acts, even if 

 it comes into place only in order to resist passive move- 

 ment. In other words, the diaphragm must at least 

 resist being sucked up into the chest during inspiration. 

 This, for the most part, covers the range of activity of 

 the diaphragm in the inspiration of most animals. 



For the most advantageous functioning of the external 

 respiratory system, it is necessary that the shoulder 

 girdle and the fore-limb should be sufficiently fixed, at 

 the moment of inspiration, to form a firm acting point 

 for the muscles which pass from them to the ribs. 



This is the condition present in the truly quadrupedal 

 animals. In these animals the muscles which, arising 

 from the fore-limbs and shoulder girdle, pass to the ribs 

 pull the movable ribs towards the fixed limbs when they 

 contract (see Fig. 49). A contracting muscle, however, 

 is like a stretched elastic band; it pulls upon both of its 

 attached ends, and will move that attachment which is 

 least firmly fixed. In a quadrupedal animal, standing 

 with its fore-limb firmly planted on the ground, the ribs 

 are pulled to the relatively fixed fore-limb, but if the fore- 

 limb be free and movable, it will be pulled towards the 

 relatively fixed ribs. This is what actually happens in 

 animals which have developed mobility of the fore-limb, 

 at the expense of its stability. It reaches its climax in 

 those animals which have completely emancipated the 

 /, fore-limb, for in these animals the muscles of the external 

 [I respiratory system have become muscles which produce 

 1 added movements of the mobile fore-limb. The mobility 

 and range of movements of the fore-limb are increased, 

 but the efficiency of the primitive respiratory mechanism 

 is impaired in proportion. It is now that the internal 



