RESPIRATION. 213 



and irresistible, rapid, and violent palpitation, and difficulty of 

 breathing, all which soon ceased on his assuming the horizontal 

 posture, in which, of course, the blood circulates more easily. 

 His guide, a slim old man, was unaffected, and climbed with 

 ease like a goat, and many unaccustomed to such elevations 

 have been equally unaffected ; for habit or a strong heart will 

 render the influence of pressure but little necessary. 



Gravity has been thought by Dr. Carson, as well as by older 

 writers, materially to aid the circulation : " By the stroke of the 

 heart, a quantity of fluid is withdrawn from one end of the 

 column, and by the synchronous vibration of the arteries an 

 equal quantity is added to the other." " A perpetually repeated 

 generation of motion must be produced through the different 

 parts of the venous system by gravity, and this motion must be 

 from the ends of the veins to the trunks." f " The simplest 

 weight of a column of blood in any descending artery is sufficient 

 to raise the blood through open capillaries to an equal height in 

 the corresponding vein, according to the hydrostatical law, that 

 fluids attain the same level in all communicating vessels." s Yet, 

 in the horizontal posture, there can be no assistance from gravity, 

 but the circulation proceeds perfectly well : and, indeed, gravity, 

 on the whole, seems to impede the circulation ; for, if the arms 

 hang down for a length of time, or the legs are not- rested 

 horizontally, they ultimately swell. Nothing assists the heart 

 more than a horizontal posture, as seen in syncope, in which the 

 restoring agency is perfectly explicable by its mechanical aid to 

 the heart, without reference to the brain. h The effects of pos- 

 ture are necessarily greater in tall persons. In the horizontal 

 posture, the heart, having less to do. beats more slowly, and in 

 very tall persons the pulse has been found 12 or 20 beats quicker 

 in the upright posture. 



The operation of exercise is very material. If an extremity is 

 not exercised, its circulation always becomes languid, it resists ex- 

 ternal temperature with difficulty, and wastes; and, if gravity also 

 co-operates by a vertical position, it swells ; and exercise will pre- 

 vent the congestive agency of a continued vertical position. Violent 

 exercise causes proportionate violence of circulation. The action 

 of muscles evidently operates by compression, and chiefly of the 

 veins, as the coats of the arteries are so much stronger. The 



f 1. c. p. 138. sq. 



. g Elements of Physics. By N. Arnott, M.D. Lond. 1827. p. 500. 

 h See Bichat, 1. c. p. 1 98. sqq. 



