A History of Science-1 [111]
the fact that the arteries also contained blood, he must have believed that these vessels contained a mixture of the two. Modern anatomists know that the heart is divided into two approximately equal parts by an impermeable septum of tough fibres. Yet, Galen, who dissected the hearts of a vast number of the lower animals according to his own account, maintained that this septum was permeable, and that the air, entering one side of the heart from the lungs, passed through it into the opposite side and was then transferred to the arteries. He was equally at fault, although perhaps more excusably so, in his explanation of the action of the nerves. He had rightly pointed out that nerves were merely connections between the brain and spinal-cord and distant muscles and organs, and had recognized that there were two kinds of nerves, but his explanation of the action of these nerves was that "nervous spirits" were carried to the cavities of the brain by blood-vessels, and from there transmitted through the body along the nerve-trunks. In the human skull, overlying the nasal cavity, there are two thin plates of bone perforated with numerous small apertures. These apertures allow the passage of numerous nerve-filaments which extend from a group of cells in the brain to the delicate membranes in the nasal cavity. These perforations in the bone, therefore, are simply to allow the passage of the nerves. But Galen gave a very different explanation. He believed that impure "animal spirits" were carried to the cavities of the brain by the arteries in the neck and from there were sifted out through these perforated bones, and so expelled from the body. He had observed that the skin played an important part in cooling the body, but he seems to have believed that the heart was equally active in overheating it. The skin, therefore, absorbed air for the purpose of "cooling the heart," and this cooling process was aided by the brain, whose secretions aided also in the cooling process. The heart itself was the seat of courage; the brain the seat of the rational soul; and the liver the seat of love. The greatness of Galen's teachings lay in his knowledge of anatomy of the organs; his weakness was in his interpretations of their functions. Unfortunately, succeeding generations of physicians for something like a thousand years rejected the former but clung to the latter, so that the advances he had made were completely overshadowed by the mistakes of his teachings.
XI. A RETROSPECTIVE GLANCE AT CLASSICAL SCIENCE It is a favorite tenet of the modern historian that history is a continuous stream. The contention has fullest warrant. Sharp lines of demarcation are an evidence of man's analytical propensity rather than the work of nature. Nevertheless it would be absurd to deny that the stream of history presents an ever-varying current. There are times when it seems to rush rapidly on; times when it spreads out into a broad--seemingly static--current; times when its catastrophic changes remind us of nothing but a gigantic cataract. Rapids and whirlpools, broad estuaries and tumultuous cataracts are indeed part of the same stream, but they are parts that vary one from another in their salient features in such a way as to force the mind to classify them as things apart and give them individual names. So it is with the stream of history; however strongly we insist on its continuity we are none the less forced to recognize its periodicity. It may not be desirable to fix on specific dates as turning-points to the extent that our predecessors were wont to do. We may not, for example, be disposed to admit that the Roman Empire came to any such cataclysmic finish as the year 476 A.D., when cited in connection with the overthrow of the last Roman Empire of the West, might seem to indicate. But, on the other hand, no student of the period can fail to realize that a great change came over the aspect of the historical stream towards the close of the Roman epoch. The span from Thales to Galen has compassed about eight hundred years--let us say thirty generations.
XI. A RETROSPECTIVE GLANCE AT CLASSICAL SCIENCE It is a favorite tenet of the modern historian that history is a continuous stream. The contention has fullest warrant. Sharp lines of demarcation are an evidence of man's analytical propensity rather than the work of nature. Nevertheless it would be absurd to deny that the stream of history presents an ever-varying current. There are times when it seems to rush rapidly on; times when it spreads out into a broad--seemingly static--current; times when its catastrophic changes remind us of nothing but a gigantic cataract. Rapids and whirlpools, broad estuaries and tumultuous cataracts are indeed part of the same stream, but they are parts that vary one from another in their salient features in such a way as to force the mind to classify them as things apart and give them individual names. So it is with the stream of history; however strongly we insist on its continuity we are none the less forced to recognize its periodicity. It may not be desirable to fix on specific dates as turning-points to the extent that our predecessors were wont to do. We may not, for example, be disposed to admit that the Roman Empire came to any such cataclysmic finish as the year 476 A.D., when cited in connection with the overthrow of the last Roman Empire of the West, might seem to indicate. But, on the other hand, no student of the period can fail to realize that a great change came over the aspect of the historical stream towards the close of the Roman epoch. The span from Thales to Galen has compassed about eight hundred years--let us say thirty generations.