OTHER VARIETIES OF MUSCULAR MOTION. 453 



In bearing a load, we have simply a variety of walking in the erect 

 attitude, with this addition, that the extensor muscles of the head, 

 neck, or back, according to the part on which the burden may be 

 placed, have to contract forcibly to support it. The position of the 

 individual has, also, to be so regulated, that the centre of gravity shall 

 always be over the base of sustentation. Hence, if the load be on a 

 man's back, he leans forward; if borne before him, he leans backward; 

 and this is the cause of the portly and consequential appearance of the 

 corpulent. If the load be on his head he stands as upright as possible, 

 for a like reason. 



In propelling a body forwards, either by the hands or shoulders, the 

 feet are firmly fixed on the ground ; the limbs are in a state of semi- 

 flexion, and the centre of gravity is directed forward, so as to aid the 

 force that has to be developed by the muscles. The limbs are then 

 suddenly extended; the body is thrown forward, and the whole power 

 exerted on the obstacle which has to be moved. 



On the other hand, when we drag a weight after us, or attempt to 

 dislodge a stake from the earth, the feet are equally fixed firmly on 

 the ground, but the body is in a state of extension, and is directed as 

 far as practicable backwards, in order that the tendency to fall, owing 

 to the centre of gravity overhanging the base of sustentation, may aid 

 the force that has to be developed by the flexor muscles of the arms, 

 which are then powerfully contracted, and the whole force is exerted 

 upon the object. As, in both these cases, there is danger of falling 

 should the body yield suddenly, the feet are so placed as to obviate 

 this, as far .as possible, by being separated in the direction in which the 

 force is exerted. 



Squeezing consists in laying hold of the object, either between the 

 arms and body, or by the fingers ; and then forcibly contracting the 

 flexor muscles. In all these, and other varieties of strong muscular 

 contraction, the respiration is interrupted, in order that the thorax 

 may be rendered fixed, and serve as an immovable point of origin for 

 the muscles of the head, shoulders, and arms. This is effected by 

 taking in a full inspiration ; strongly contracting the respiratory mus- 

 cles, and, at the same time, closing the glottis to prevent the exit of 

 the air. 



Lastly : as organs of prehension, the upper extremities are of admi- 

 rable organization, possessing great mobility, and at the same time 

 solidity. The joint at the shoulder allows of extensive motion ; and 

 the bones, to which the arm is attached at this joint scapula and 

 clavicle are themselves movable. The forearm is likewise susceptible 

 of various movements on the arm, of which those of pronation and 

 supination are not the least important ; whilst the hand possesses every 

 requisite for an organ of prehension. It is composed of numerous 

 bones, and is capable of being applied to the most irregular surfaces. 

 The great superiority of the human hand arises from the size and 

 strength of the thumb, which can be brought into a state of opposition 

 to the fingers ; and is, therefore, of the highest use in enabling us to 

 seize hold of, and grasp spherical bodies; to take up any object; to 

 lay firm hold of whatever we seize, and to execute the various useful, 



