GENERAL USES OF THE CIRCULATION. 179 



remedy seems to consist in its restoring the contractility of 

 the small arteries under the influence of the vaso-motor 

 nerves proceeding from the great sympathetic. Digitalis 

 must, therefore, be henceforward considered as regulating 

 the circulation by means of an exciting, tonic action, and 

 not a hyposthenisant as is generally supposed (Ilirtz, "Nouv. 

 Diet, de Med. et de Chirurgie"). 



IV. GENERAL USES OF THE CIRCULATION. 



THE principal purpose of the circulation is to produce 

 rapid currents in the interior of the tissues, intended to sup- 

 ply the organs with the materials of nutrition, and to carry 

 off the waste resulting from the changes which these under- 

 go, as we pointed out at the beginning of our description of 

 the organism. These changes take place in the capillaries ; 

 we know that the pressure in these small vessels is generally 

 from T^Q to y 1 ^ of the atmosphere, and this pressure appears 

 very favorable to the regularity of the changes. When the 

 pressure is diminished, as after bleeding, reabsorption takes 

 place ; if, on the contrary, the pressure in the capillaries is 

 increased, as by compression or ligature of a vein, the exu- 

 dation exceeds the normal limits, and the serum of the blood, 

 overflowing into the tissues, constitutes what is called 

 oedema. Paralytic dilatation of the small arteries may also 

 produce oedema by increasing the afflux of blood, and, conse- 

 quently, the pressure in the capillaries. 1 The stings of 



1 According to a recent communication by Ranvier to the Aca- 

 demie des Sciences (January, 1870), vaso-motor paralysis is the most 

 important condition in the production of oederaa. In attempting 

 to produce artificial oedema by compression and obliteration of the 

 veins, Ranvier was surprised to find that the ligature never caused 

 serous infiltration in the parts situated beyond it. 



In experimenting on rabbits and dogs, he tied, first, the two 

 jugular veins at the base of the neck; second, the femoral vein, at 

 the level of the crural ring; third, the inferior vena cava. In none 

 of these cases did oedema ensue, either in the face or the lower 

 limbs; while, having cut the sciatic nerve on one side, thus para- 

 lyzing the vaso-motor nerves of the limb, in a dog in which he had 

 tied the inferior vena cava, he found that considerable oedema fol- 

 lowed on this side, and the other remained in its normal condition. 

 The same phenomenon was repeated in several experiments. 



It is true that the sciatic nerve is a mixed nerve, containing in 

 the same covering sensitive fibres, voluntary motor fibres, and vaso- 



