40 HORSE-BACK RIDING. 



to the heart, from which it again at once departs to 

 repair the losses which the arterial blood has suffered, 

 and to cause a new muscular contraction. The 

 whole system is in action, and the blood penetrates 

 abundantly and fills the entire vascular system, where 

 before it entered insufficiently and with difficulty. 



It is interesting to study, first of all, the manner in 

 which the blood acts in the capillaries. A knowledge 

 of the facts will enable us to explain a number of 

 phenomena which we shall notice further on. 



In the structure of the vessels which form the con- 

 tinuations of the smaller arteries, muscular fibre pre- 

 dominates. The play of these fibres, which differ 

 essentially in their anatomical elements from those 

 which we have heretofore studied, is independent of 

 the will. It is governed by a special system of 

 nerves, springing from the grand sympathetic and 

 also some of the spinal nerves — the vaso-motors — and 

 according as these nerves are in action or inaction the 

 capillaries contract and dilate. The fibre cells are 

 not found in the minutest ramifications of these ves- 

 sels, but they are replaced by an elastic tissue which 

 follows the modifications of the muscular tissue and 

 contracts or augments the calibre of the canals which 

 it forms. Moral as well as physical impressions cause 

 important modifications in the circulation of the 

 capillaries. Every one is cognizant of the flush and 

 the paleness which accompany anger or shame, pain 

 or pleasure. But the vaso-motors are yet much more 



