The white matter (WM) of the adult human neocortex contains the so-called "interstitial neurons" . They are most numerous in the superfi cial WM underlying the cortical gyri, and decrease in density toward the deep WM. They are morphologically heterogeneous. A subgroup of interstitial neurons display pyramidal-cell like morphologies, characterized by a polarized dendritic tree with a dominant apical dendrite, and covered with a variable number of dendritic spines. In addition, a large contingent of interstitial neurons can be classifi ed as interneurons based on their neurochemical profi le as well as on morphological criteria. WM-interneurons have multipolar or bipolar shapes and express GABA and a variety of other neuronal markers, such as calbindin and calretinin, the extracellular matrix protein reelin, or neuropeptide Y, somatostatin, and nitric oxide synthase. The heterogeneity of interstitial neurons may be relevant for the pathogenesis of Alzheimer disease and schizophrenia. Interstitial neurons are most prominent in human brain, and only rudimentary in the brain of non-primate mammals. These evolutionary differences have precluded adequate experimental work on this cell population, which is usually considered as a relict of the subplate, a transient compartment proper of development and without a known function in the adult brain. The primate-specifi c prominence of the subplate in late fetal stages points to an important role in the establishment of interstitial neurons. Neurons in the adult WM may be actively involved in coordinating inter-areal connectivity and regulation of blood fl ow. Further studies in primates will be needed to elucidate the developmental history, adult components and activities of this large neuronal system.

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Arsenic (As) was isolated for the first time as a chemical element by Jabir-Ibn-Haiyanb in the year 776, and its name comes etymologically from the Greek arsenicon (masculine). It is gray in color and of a shining metallic appearance, has little metallic properties, is distributed widely in the universe and its toxicity depends on the different chemical forms and states of oxidation that it has:-3, 0, +3 and +5. The historical evolution of the toxicology is intimately united to the use of this nonmetal. At present, arsenical substances are used in industry, agriculture, cattle ranching, and medicine. For that reason, the sources of exposure of man to this metal are diverse; out standing are labor, food and medicaments. In this work a revision is made of the etiology, preventive measures, analysis, diagnosis and treatment of the poisoning by As.