MOVEMENT OF BLOOD IN THE VEINS. 339 



fluence in the act of blushing, which consists in a sudden enlargement 

 of the capillaries and small vessels of the surface ; whilst the converse 

 state of pallor, which often alternates with it under the influence of 

 strong emotion, is evidently due to an unusual contraction of the same 

 vessels. But the effects of this influence are no less sensible in other 

 cases ; and particularly in the regulation of the quantity of certain 

 secretions, in accordance with the mental state, or the condition of the 

 system generally. To the mode in which this regulation is effected, the 

 act of blushing seems to afford us the key ; for it indicates that the 

 supply of blood afforded to the glands, may be entirely governed by the 

 influence of the nervous system upon the calibre of the arteries. Thus 

 the nursing mother, at the sight, or even at the thought, of her child, 

 when the usual time for suckling approaches, feels a rush of blood to 

 the breast, exactly resembling that which takes place to the cheeks in 

 blushing, and popularly termed "the draught ;" this rush occasions an 

 almost immediate increase in the secretion. In like manner we may 

 explain the influence of the mental state upon the amount of the secre- 

 tions of the lachrymal, the salivary, and many other glands ; its influence 

 upon their quality, must probably be effected through changes in the 

 condition of the blood itself. 



604. The supply of Nervous agency from the Cerebro-spinal system, 

 has been clearly proved to exert no direct influence in maintaining the 

 capillary circulation ; since the latter continues as usual, after all the 

 nerves of a part have been divided. This is obviously due to the fact, 

 that the operations of nutrition, secretion, &c., are essentially indepen- 

 dent of this agency. But as they are in some degree influenced by it, 

 so will the capillary circulation be affected, through its connexion with 

 them. In this manner we are to explain the effect of violent impres- 

 sions upon the nervous centres in bringing to a stand, not merely the 

 action of the Heart ( 581), but the Capillary circulation all over the 

 body. The general vitality of the system appears to be at once de- 

 stroyed ; so that the capillary circulation, which may usually be seen to 

 continue in the web of a frog's foot for some time after the interruption 

 of the heart's action, is immediately suspended by crushing the brain 

 with a hammer. 



6. Of the Movement of Blood in the Veins. 



605. The Venous system is formed by the reunion of the small trunks, 

 which originate in the capillary network ; and it carries -back to the 

 heart the "blood which has been transmitted through this. This blood is 

 dark or carbonated in the systemic veins ; whilst it is bright or oxy- 

 genated in the pulmonary veins. The structure of the veins is essen- 

 tially the same with that of the arteries ; but the fibrous tissue of their 

 middle coat less decidedly exhibits the characters, either of the yellow- 

 elastic tissue, or of non-striated muscle. Still it possesses no inconside- 

 rable amount of Elasticity ; and a certain degree of Muscular contrac- 

 tility also. The whole capacity of the Venous system is at least twice, 

 and perhaps more nearly three times, that of the Arterial; and the 

 rate of motion of the blood in it must be proportionably slower. 



