<P> Psychopathology is the study of mental illness, particularly of severe disorders . Informed heavily by both psychology and neurology, its purpose is to classify mental illness, elucidate its underlying causes, and guide clinical psychiatric treatment accordingly . Although diagnosis and classification of mental norms and disorders is largely the purview of psychiatry--the results of which are guidelines such as the Diagnostic and Statistical Manual of Mental Disorders, which attempt to classify mental disease mostly on behavioural evidence, though not without controversy--the field is also heavily, and increasingly, informed upon by neuroscience and other of the biological cognitive sciences . Mental or social disorders or behaviours seen as generally unhealthy or excessive in a given individual, to the point where they cause harm or severe disruption to the sufferer's lifestyle, are often called "pathological" (e.g., pathological gambling or pathological liar). </P> <P> Although the vast majority of lab work and research in pathology concerns the development of disease in humans, pathology is of significance throughout the biological sciences . Two main catch - all fields exist to represent most complex organisms capable of serving as host to a pathogen or other form of disease: veterinary pathology (concerned with all non-human species of kingdom of Animalia) and phytopathology, which studies disease in plants . </P> <P> Veterinary pathology covers a vast array of species, but with a significantly smaller number of practitioners, so understanding of disease in non-human animals, especially as regards veterinary practice, varies considerably by species . Nonetheless, significant amounts of pathology research are conducted on animals, for two primary reasons: 1) The origins of diseases are typically zoonotic in nature, and many infectious pathogens have animal vectors and, as such, understanding the mechanisms of action for these pathogens in non-human hosts is essential to the understanding and application of epidemiology and 2) those animals that share physiological and genetic traits with humans can be used as surrogates for the study of the disease and potential treatments as well as the effects of various synthetic products . For this reason, as well as their roles as livestock and companion animals, mammals generally have the largest body of research in veterinary pathology . Animal testing remains a controversial practice, even in cases where it is used to research treatment for human disease . As in human medical pathology, the practice of veterinary pathology is customarily divided into the two main fields of anatomical and clinical pathology . </P> <P> Although the pathogens and their mechanics differ greatly from those of animals, plants are subject to a wide variety of diseases, including those caused by fungi, oomycetes, bacteria, viruses, viroids, virus - like organisms, phytoplasmas, protozoa, nematodes and parasitic plants . Damage caused by insects, mites, vertebrate, and other small herbivores is not considered a part of the domain of plant pathology . The field is deeply connected to plant disease epidemiology and the horticulture of species that are of high importance to the human diet or other uses . </P>

The term that means between but not within the parts of a tissue