THE CIRCULATION OF THE BLOOD. 289 
motor center, situated in the medulla, and possibly certain sub- 
ordinate centers in the spinal cord, through vaso-motor nerves. 
These are (a) vaso-constrictors, which maintain a constant but 
variable degree of contraction of the muscle-cells of the vessels; 
(6) vaso-dilators, which are not in constant functional activity ; 
and (c) mixed nerves, with both kinds. An inherited tendency 
to rhythmical contraction throughout the entire vascular sys- 
tem, including the vessels, must be taken into account. 
The depressor nerve acts by lessening the tonic contraction 
of (dilating) the vessels of the splanchnic area especially. 
It is important to remember that all the changes of the 
vascular system, so long as the nervous system is intact—i.e., 
so long as an animal is normal—are correlated; and that the 
action of such nerves as the depressor is to be taken rather as 
an example of how some of these changes are brought about, 
mere chapters in an incomplete but voluminous history, if we 
could but write it all. The changes in blood-pressure, by the 
addition or removal of a considerable quantity of blood, are 
slight, owing to the sort of adaptation referred to above, effected 
through the nervous system. Finally, the capillary circulation, 
when studied microscopically, and especially in disordered con- 
ditions, shows clearly that the vital properties of these vessels 
have an important share in determining the character of the 
circulation in themselves directly and elsewhere indirectly. 
The study of the circulation in other groups shows that 
below birds the arterial and venous blood undergoes mixture 
somewhere, usually in the heart, but that in all the vertebrates 
the best blood is invariably that which passes to the head and 
upper regions of the body. The deficiencies in the heart, owing 
to the imperfections of valves, septa, etc., are in part counter- 
acted in some groups by pressure relations, the blood always 
flowing in the direction of least resistanoe, so that the above- 
mentioned result is achieved. 
Capillaries are wanting in most of the invertebrates, the 
blood flowing from the arteries into spaces (sinuses) in the tis- 
sues. It is to be noted that a modified blood (lymph) is also 
found in the interspaces of the cells of organs. Indeed, the 
circulatory system of lower forms is in many respects analogous 
to the lymphatic system of higher ones. 
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