THE SENSORI-MOTOR SYSTEM 17 



the lungs and heart. They can be raised up by muscles, and as 

 they are inclined downwards the upward movement enlarges the 

 diameter of the thorax. This is the mechanism of respiration. 



The Limb Girdles. The two pairs of limbs, which are charac- 

 teristic (as fins, paddles, legs and wings, fore and hind limbs, 

 arms and legs) of all vertebrates, are not attached directly to the 

 backbone, but to the two limb girdles. Each of the latter is a 

 kind of hoop attached to the column. The shoulder girdle 

 consists of the two collar-bones, which are attached in front to 

 the breast- bone and behind to the shoulder-blades. The latter 

 are not attached to the column, but are moored, so to speak, 

 in the muscles of the back. The skeletons of the fore-limbs 

 are attached to the 

 shoulder-blades. 



The pelvic girdle 

 also consists of 

 several bones, and in 

 man these are fused 

 together to form the 

 two hip-bones, which 

 are immovablv at- FIG. 2. DIAGRAM OF THE VERTEBRAL COLUMN, 

 tached to the column ^ MB G> AND LIMBS IN A BACKBONED 

 , , . , , . . , ANIMAL. 

 behind, and are joined 



together in front at the pubis. The skeletons of the hind-limbs 

 are jointed into the pelvic bones, but the latter, together, also 

 act as a kind of basin-shaped support for the lower bowel, the 

 urinary bladder, and the reproductive organs. 



Note that the articulation of the limbs with the limb girdles 

 means that the former are set well out from the axis of the body, 

 so that the feet rest on the ground on as wide a base as possible. 



The Limb Skeleton. All the higher animals that is, the 

 vertebrates, the Crustacea (such as the lobster, crab, or prawn), 

 the insects and spiders possess true limbs, which are organs of 

 locomotion, and are also bodily weapons. The nature of these 

 limbs is, of course, very different in the various groups of higher 

 animals, but in all those that we have mentioned they are jointed 

 appendages of the body, freely movable on the latter, and as a 

 rule furnished with weapons. The limb may be a walking leg 

 (as in the case of the fore and hind legs of most quadrupeds), a 

 fin (as in the case of a fish), a paddle (in seals, dolphin, and 





