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Brain   

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exoplanet

The smiths that produced the high-quality blades would most likely have kept the process for making these blades a closely guarded secret to be passed on only to their apprentices. The smiths would be able to teach the apprentices the second and third points listed, but point one is something they would not have known. There is no difference in physical appearance between an ingot with the proper minor elements present and one without. Suppose that during several generations all of the ingots from India were coming from an ore body with the proper amount of minor elements present, and blades with good patterns were being produced. Then, after a few centuries, the ore source may have been exhausted or become inaccessible to the smithing community; therefore, the technique no longer worked. With time, the smiths who knew about the technique died out without passing it on to their apprentices (since it no longer worked), so even if a similar source was later found, the knowledge was no longer around to exploit it. The possible validity of this theory could be examined if data were available on the level of carbide-forming elements in the various ore deposits in India used to produce wootz steel.

And in truth, I am quite willing it should be known that the little I have hitherto learned is almost nothing in comparison with that of which I am ignorant, and to the knowledge of which I do not despair of being able to attain; for it is much the same with those who gradually discover truth in the sciences, as with those who when growing rich find less difficulty in making great acquisitions, than they formerly experienced when poor in making acquisitions of much smaller amount. Or they may be compared to the commanders of armies, whose forces usually increase in proportion to their victories, and who need greater prudence to keep together the residue of their troops after a defeat than after a victory to take towns and provinces. For he truly engages in battle who endeavors to surmount all the difficulties and errors which prevent him from reaching the knowledge of truth, and he is overcome in fight who admits a false opinion touching a matter of any generality and importance, and he requires thereafter much more skill to recover his former position than to make great advances when once in possession of thoroughly ascertained principles. As for myself, if I have succeeded in discovering any truths in the sciences (and I trust that what is contained in this volume 1 will show that I have found some), I can declare that they are but the consequences and results of five or six principal difficulties which I have surmounted, and my encounters with which I reckoned as battles in which victory declared for me. I will not hesitate even to avow my belief that nothing further is wanting to enable me fully to realize my designs than to gain two or three similar victories; and that I am not so far advanced in years but that, according to the ordinary course of nature, I may still have sufficient leisure for this end.

Though artisans may not be able at once to execute the invention which is explained in the "Dioptrics," I do not think that any one on that account is entitled to condemn it; for since address and practice are required in order so to make and adjust the machines described by me as not to overlook the smallest particular, I should not be less astonished if they succeeded on the first attempt than if a person were in one day to become an accomplished performer on the guitar, by merely having excellent sheets of music set up before him.

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The Ch1-4 nomenclature was introduced to designate cholinergic (i.e., ChAT-containing) neurons within four overlapping cell groups of the basal forebrain (Mesulam and Geula, 1988; Mesulam et al., 1983a,b). In this nomenclature, Ch 1 designates the cholinergic cells associated predominantly with the medial septal nucleus, Ch2 those associated with the vertical limb of the diagonal band, Ch3 those associated with the horizontal limb of the diagonal band, and Ch4 those associated with the nucleus basalis of Meynert. In the macaque monkey, the Ch4 group is by far the largest of the basal forebrain cholinergic cell groups by volume and number of neurons and has been subdivided into several distinct sectors (Fig. 2).

Ch1-Ch2 compartment (Figs. ​(Figs.1A,1A, 2A-B) Magnocellular neurons at the level of the area subcallosa and gyrus rectus stretching from just below the fused midline of the septum to the ventral edge of the brain and extend close to the medial border of the nucleus accumbens. This compartment largely corresponds to the medial septal nucleus and the nucleus of the diagonal band of Broca as defined by Andy and Stephan (1968). In the more dorsal part of this area, corresponding to the Ch1 group of Mesulam et al. (1983), the cell bodies and initial dendrites of the neurons tend to be oriented perpendicularly to the crossing fibers of the anterior commissure in coronal sections. The ventral part of this compartment (Ch2 group of Mesulam) merges laterally with the Ch3 cell groups. No distinction was made between Ch1 and Ch2 groups.

Delination of Ch1 and Ch2 nuclei Every third section was used for delineation of the boundaries of the Ch1 and Ch2 (septal) nuclei. These two nuclei are the only portions of the magnocellular basal complex located within septum verum. Septum verum is well developed in the human brain contrary to the dorsally located septum pellucidum (Andy and Stephan, 1968). Figure 1 is provided as an example to illustrate the topography of the Ch1 and Ch2 nuclei within the median septum verum as well as their size, shape and cytoarchitecture. In cresyl violet staining Ch1 and Ch2 neurons stand out sharply from their surroundings due their size and intense staining of their Nissl material. CH1 is composed mainly of small to medium size round neurons, whereas CH2 is composed mainly of larger hyperchromatic neurons with prominent nucleus and dark nucleolus. Ch1 (medial septal nucleus with approximately 10% of cholinergic neurons) boundaries are defined by small-sized neurons in the septum verum (dorsal and lateral border) and the diagonal band of Broca (ventral border). Dorsally, the Ch2 group (vertical limb nucleus of the diagonal band of Broca with approximately 70% of cholinergic neurons) borders with Ch1, medially with subarachnoidal space, and laterally with nucleus accumbens. Caudally Ch2 extends to the level of the globus pallidus (GP) (Vogels et al., 1990). Because the major portion of CH2 is detected at the level of the GP, the anterior GP extent – which is typically quite distinct on MRI – was chosen as the anterior landmark for beginning to trace septal nuclei on coronal MRI. Because septal nuclei borders the wall of lateral ventricle (see Figure 1), we chose a vertical line through the most medial border of each lateral ventricle as an MRI-visible lateral boundary. See below for details of manual tracing protocol.

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