442 MUSCULAR MOTION. 



are allowed to touch also, by the points of the toes, the attitude is much 

 more easy and firm, as the base of sustentation is largely augmented, 

 and comprises the space between the knees and toes plus the space 

 occupied by those parts. 



The sitting posture admits of variety, and its physiology is easily 

 intelligible. In every form in which the back is unsupported, the 

 weight of the body is conveyed to the soil by the pelvis; and the broader 

 this base the firmer the attitude. When we sit upon a stool without any 

 back, and with the legs raised from the ground, the whole of the weight 

 is conveyed by the parts in contact with the seat; but if the feet touch 

 the ground, the weight of the lower extremities is transmitted to the 

 soil by the feet, whilst the pelvis transmits that of the upper part of 

 the body. In both cases, if the attitude be long maintained, fatigue is 

 felt in the back, owing to the continued action of the extensor muscles 

 in keeping the body erect. Sitting in an ordinary chair differs some- 

 what, in a part of the body being supported. Fatigue is felt in the 

 neck, which is unsupported, and requires the sustained contraction of 

 the extensor muscles of the head. To support all the parts, as far as 

 possible, long-backed chairs have been introduced, which sustain the 

 whole body and head; and, when they are provided with rockers, a 

 position approaching to the easiest of all attitudes can be assumed. To 

 produce a similar effect in a common chair, the body is often thrown 

 back until the chair rests on its hinder legs only. When the feet of 

 the individual are on the ground, this position is stable ; the base of 

 sustentation being large, and comprised between the legs of the chair 

 and the feet of the individual, added to the space occupied by the parts 

 themselves, that are in contact with the soil ; but as soon as he raises 

 his feet, the equilibrium is destroyed from the impracticability of mak- 

 ing the vertical line fall within the base of sustentation, which is now 

 reduced to the space occupied by the legs of the chair plus the space 

 between them. In all the varieties of the sitting posture, equilibrium 

 is facilitated by the centre of gravity being brought nearer to the 

 ground. 



Lastly. The horizontal posture is the only one that requires no mus- 

 cular effort. Hence it is the attitude of repose, and of the sick and 

 the feeble. The base of sustentation is here extremely large ; and the 

 centre of gravity very low. Accordingly, the attitude can be maintained 

 for a long time; the only inconvenience being that which results to the 

 skin from prolonged pressure on those parts that chiefly convey the 

 weight to the bed, as the back of the pelvis, the region of the great 

 trochanter, &c. an inconvenience, which attracts the attention of the 

 physician, more or less, in all protracted and consuming maladies. 

 The reason, why we prefer soft, elastic beds, is not simply to prevent 

 abrasion of those parts of the body that are most exposed to pressure, 

 but to enable a greater portion of the body to transmit the weight; and 

 thus occasion a more equable partition of the pressure. 



There are numerous other attitudes, which maybe assumed; as, upon 

 one knee, on the head, astride, &c. ; but they do not need explanation, 

 their physiology being obvious after what has been said. 



