collect it from the capillaries and return it to the heart. There are two distinct sets of blood-vessels in the body, both connected with the heart; one set carries blood to, through and from the lungs, the other guides its flow through all the remaining organs; the former are known as the pulmonary, the latter as the systemic blood-vessels.
The smallest arteries pass into the capillaries, which have very thin walls, and form very close networks in nearly all parts of the body; their immense number compensating for their small size. It is while flowing in these delicate tubes that the blood does its nutritive work, the arteries being merely supply-tubes for the capillaries, through whose delicate walls liquid containing nourishment exudes from the blood to bathe the various tissues.
The quantity of blood in any part of the body at any given time is dependent upon certain relations which exist between the blood-vessels and the nervous system. The walls of the arteries are abundantly supplied with involuntary muscular fibres, which have the power of contraction and relaxation. This power of contraction and relaxation is controlled by certain nerves called vasomotor nerves, because they cause or control motion in the vessels to which they are attached. When arteries supplying blood to any particular part of the body contract, the supply of blood to that part will be diminished in proportion to the amount of contraction. If the nervous control be altogether withdrawn, the
