The brain and nerves were also the subjects of his researches. He described the cortical substance as being entirely vascular, and said that the vessels which compose it are five hundred and twelve times smaller than the minutest capillaries; and that the globules which compose the fluid contained In these vessels are thirty-six times more minute than those which form the red blood. Fresh experiments made him change his opinions, and in 1717 he showed that the brain and nerves are fibrous structures, and that the blood-vessels glide between the fibres which compose these tissues. These observations very nearly agree with those of modern anatomists as to the structure of the brain ; the only part in which Leuwenhoek seems to have beau deficient was in a clear knowledge of the difference of structure between the cortical or grey and the medullary or white parts of the brain. Thus when he discovered that the latter was fibrous, he supposed that the former must be so also; whereas tho cortical substance is composed almost entirely of blood-vessels connected by exceedingly fine cellular membrane, as first stated by Leuwanhoek, and iuveating, as has been since ascertained by Valentin, small grey globules or granules. It is now universally agreed that the medullary part of tho brain is composed of fibres.
Leuwenhoek examined the structure of the crystalline lens, and described with exactness the disposition of the layers which compose this part of the organ of vision; and he embellished his description with several very good figures.
Much has been said conceroing his investigation of the well-known and celebrated spermatic animalcules, which since the time of their first discovery in 1677 have excited the curiosity and speculative fancy of many naturalists. Haller states that Ludwig Hamm (a student at Leyden) was the first discoverer of the seminal animalcules, in Augnet 16i 7. Lenwenhoek, who minutely described them, claimed the merit of having made the discovery in the November of the same year; and in 1678 Hartscelter published an account of them, in which he professed to have seen them as early as 1674. A great deal has since been written upon them by Needman, Buffon, Der Glelchen, Spallauzani, Prevost and Dumas (their experiments were made together), Wagner, and others.
Leuwenhoek would have made both more numerous and more valuable discoveries, if he had possessed greater erudition, which would have enlarged his ideas, and prevented him from mistakingias Ile did in some instances, probabilities for facts. Thus he often fancied that he saw what did not exist, and afterwards he persisted in his error. Among other mistakes he considered that the villous or mucous coat of the intestines was muscular; he also maintained that pulsation belonged to veins, and not to arteries.
Leuwenboek'e reputation was very extensive. When Queen Mary was in Holland, she paid him a visit, and she was highly delighted with his curiosities. He presented her with two of his microscopes. When the Czar Peter the Great was passing through Delft in 1698, he sent two of his attendants to request Leuwenhock to pay him a visit, and to bring his microscope with him. The philosopher, after having shown his instruments to the emperor, exhibited to him the curious phenomenon of the circulation of the blood in the tail of an eel.
Lenwenhoek died at Delft in 1723. Besides hie contributions to the 'Philosophical Transactions,' be published about 26 papers in the Memoirs of the Academy of Sciences.' His writings were collected and published separately in Dutch at Delft and Leyden ; they were also translated for him into Latin, and printed at Delft, in 4 vols. 4to, in 1695.99. An English translation was made from the Dutch and Latin editions in 1798-1800, by Mr. Samuel Hoole, ini4to. At his death he bequeathed to the Royal Society of London a small Indian cabinet, in the drawers of which were contained thirteen little boxes or cases, each holding two microscopes handsomely mounted with silver, of which not only the lenses but the whole apparatus were made with his own hands; each microscope had an object placed before it, of which there was an accompanying drawing made by himself.
(Philosophical Transactions for 1723; Biographic Ustivereelk, &c.)