View of the manner in which the minute branches of the
vein unite to form the larger branches and the trunks.
1. Capillary venous branches; 2. small branches formed by
the union of the capillary; 3. larger branches formed by
the union of the smaller and gradually increasing in size,
to form the great trunk, 4. a portion of which is laid open
to show its inner surface and the arrangement of 5. the
valves formed by its inner coat.
276. The veins are very much more numerous than the arteries, for they often consist of double sets, and they are at the same time more capacious and more extensible. Reckoning the whole of the blood at one-fifth of the weight of the body, it is estimated that, of this quantity, about one-fourth is in the arterial and the remaining three-fourths in the venous system. The combined area of the branches of the veins is much greater than that of the two trunks in which they terminate (fig. CXX. 1, 2, 3, 4): the blood, therefore, in returning to the heart, is always flowing from a large into a smaller space.
277. The divisions and subdivisions of the artery freely communicate in all parts of the body by means of what are called anastomosing branches, and this communication of branch with branch and trunk with trunk is termed anastomosis. The same intercommunication, but with still greater freedom and frequency, takes place among the branches of veins. In both orders of vessels the communication is frequent in proportion to the minuteness of the branch and its distance from the heart. It is also more frequent in proportion as a part is exposed to pressure; hence the minute arteries and veins about a joint are distinguished for the multitude of their anastomosing branches; and above all, it is frequent in proportion to the importance of the organ; hence the most remarkable anastomosis in the body is in the brain. By this provision care is taken that no part be deprived of its supply of blood; for if one channel be blocked up, a hundred more are open to the current, and the transmission of it to any particular region or organ by two or more channels, instead of through one trunk, is a part of the same provision. Thus the fore-arm possesses four principal arteries with corresponding veins, and the brain receives its blood through four totally independent canals[6].
278. That the blood is really a flowing stream, and that it pursues the course described (258), is indubitable. For,
(1.) With the microscope, in the transparent parts of animals, the blood can be seen in motion (fig. CXXI.); and if its course be attentively observed, its route may be clearly traced.
View of the circulation of the blood as seen under the
microscope in the web of the frog's foot.