674 SPECIAL PHYSIOLOGY. 



themselves, by their elasticity, to the varying quantities of blood, dis- 

 tributed to any particular part. Such conditions must occur in the 

 opposite states of blushing and pallor of the skin; in the conditions of 

 fulness or emptiness of the capillaries of a gland or membrane, accord- 

 ing as it is secreting or not; and in the condition of health and inflam- 

 mation, in any vascular part, such as the conjunctiva of the eyeball. 



The real propulsive cause of the motion of the blood in the capilla- 

 ries, is the same as that of the arterial circulation, viz., the ventricular 

 systole, modified, in its effects, by the resilience of the elastic coat 

 of the arteries themselves. Indeed, in a living animal, if the force of 

 the left ventricle, communicated to the blood in the arteries, be arrested 

 by pressure on, or ligature of, those vessels, the stream of blood in 

 the capillaries soon almost entirely stagnates, and the venous current 

 beyond them is stopped, whilst the tension or blood pressure in the 

 arteries also ceases. Moreover, in the fish, as we shall hereafter find, 

 the force of the single ventricle of the heart, is sufficient to propel the 

 blood first through the gills, and then through the arteries of the body. 

 It has been supposed that certain mutual attractions and repulsions 

 between the blood and the tissues lying outside the systemic capillaries, 

 or between the blood and the air in the lungs, may influence the move- 

 ment of the blood through the capillaries, and even constitute a moving 

 power in the capillary circulation. But this implies an attractive force 

 in regard to the blood in the arterial half of the capillary network, 

 and a repulsive force in regard to the blood in the venous half of that 

 network, an hypothesis complex, and yet unproved. 



The supposition of the existence of a local attractive and propulsive 

 force, exerted on the blood passing through the capillaries, is held to 

 be supported by the following facts: The gradual emptying of the ar- 

 teries after death; the maintenance of the circulation in the portal 

 system ; the periodic and local changes in the circulation during secre- 

 tion, or in other conditions, such as fainting and fright, or in diseases 

 such as congestion and inflammation; the obstructive changes which 

 occur in the pulmonary circulation during asphyxia; the great activity 

 of this circulation, when the respiratory changes are rapid; and lastly, 

 the fact of a circulation of blood occurring in the embryo of animals 

 before the development of the heart (Draper). It has been further 

 pointed out, that of two fluids contained in a capillary tube, that which 

 has the greatest affinity for the sides of the tube, will flow along it 

 quicker than the other, owing to mere physical action. 



On the other hand it is alleged that although a healthy condition. of 

 the walls of the capillaries and of the tissues beyond them, and a healthy 

 performance of their functions, are necessary to an unimpeded flow of 

 the blood through these vessels, and although a stasis or stagnation of 

 the blood, and a dilatation of the capillaries, accompany a state of in- 

 flammation, imperfect secretion, or defective respiration; yet such facts 

 do not prove the existence of a special propulsive force, resident in the 

 walls of the healthy capillaries, or dependent upon the healthy nutri- 

 tive, secernent, or respiratory function ; they may merely show that 

 the capillary circulation, though dependent upon the action of the heart 

 and arteries, may be retarded or arrested by abnormal relations be- 



