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This integration disclosed by comparisons Post Date: Wed, 13 Aug 2008 15:52:35 +0000 When from limbless Annelids we pass to the Articulate types, composed of seg- ments bearing limbs, we find the nervous system formed of a series of centres, each sending fibres to the different organs of its own segment, and all of them united by a thick cord of fibres with a fused cluster of similar centres in the head. In the higher Articulata there is an increased relative size of the nervous centres as compared with their connecting structures ; an actual approach of the chief nervous centres to one another, both longitudinally and laterally ; and a final coalescence of them. This integration disclosed by comparisons of lower and higher types, may also be observed in progress during the development of the individual insect or the individual crustacean. Autor of the post: Undefined Its lowest known member Post Date: Wed, 13 Aug 2008 15:39:57 +0000 And along with advancing growth, consolidation, and combination of nervous structures, there may be traced an increasing un- likeness, both among the central masses themselves, among their connecting cords, and among their divergent fibres. Such traits of evolution are exhibited under another form in the vertebrate sub-kingdom. Its lowest known member the AmpJiioxus, has a simple cranio-spinal axis, the anterior extremity of which is not made appreciably different from the rest by development of distinct cerebral ganglia, and which gives off lateral nerves that have but minor dis- similarities. Autor of the post: Undefined But in the highest fishes Post Date: Wed, 13 Aug 2008 15:22:41 +0000 The cyclostome Fishes, possessed of cerebral ganglia that are tolerably manifest, lead us to the ordinary fishes, in which these ganglia, individually much larger, form a cluster of masses, or rudimentary brain. Here, however, though in contact, they preserve a serial arrangement : their aggregation is little more than that of close linear succession. But in the highest fishes certain of them which have greatly increased, overlap the others ; and tend so to form a more compact, as well as a larger, aggregate. Autor of the post: Undefined From them upwards, the leading Post Date: Wed, 13 Aug 2008 15:05:23 +0000 Superior Eeptiles and Birds display this relative increase of certain of the clustered ganglia, and con- sequent obscuration of the rest, in a greater degree. It is carried still further in the inferior Mammals. From them upwards, the leading change of nervous structure is an augmentation of the two largest pairs of these aggregated nervous centres. Autor of the post: Undefined These are both longitudinal Post Date: Wed, 13 Aug 2008 14:49:41 +0000 In Man one pair has become so enormous that the others are most of them hidden by it, and nearly merged in it. Along with this direct inte- gration there goes on the indirect integration constituted by more intimate and multiplied connections. These are both longitudinal and transverse. Autor of the post: Undefined Meanwhile there have been arising Post Date: Wed, 13 Aug 2008 14:34:52 +0000 While in the Amphioxus, the cranio-spinal axis contains but a small proportion of the nerve-fibres which, running longitudinally, serve to unite its different parts ; in a superior vertebrate animal, such uniting nerve-fibres are among the chief com- ponents of the cranio-spinal axis. And, similarly, while the lateral halves of the cerebrum are but slightly connected in Birds, and have connections that are relatively deficient in the inferior Mammals, they become, in the highest Mammals, joined together by a thick mass formed of innumerable fibres. Meanwhile there have been arising diffe- rentiations no less conspicuous. Autor of the post: Undefined The same thing holds Post Date: Wed, 13 Aug 2008 14:23:22 +0000 Beyond that general one due to development of the anterior end of the cranio- spinal axis into cerebral ganglia; and the further one of like nature which results from the relatively enor- mous growth of some of these ; other differentiations have been constituted by the local unlikenesses of structure simultaneously established. As they enlarge, the greater ganglia are rendered externally dissimilar from the rest by the formation of folds or convolutions j and their internal parts severally acquire distinctive characters. The same thing holds of the peripheral nervous system. Autor of the post: Undefined We are also shown Post Date: Wed, 13 Aug 2008 14:11:38 +0000 Pairs Of nerves that were originally almost uniform, are rendered multiform by the much greater growth of some than of others, and by the inner differences that accompany these outer differences. This cursory survey of the nervous system under the various forms it presents throughout the animal kingdom, suffices to show how its evolution conforms to the laws of evolution in general. We are also shown by it what here more immediately concerns us that while the rudimentary nervous system, consisting of a few threads and minute centres, is very much scattered, its increase of relative size and increase of complexity, go hand in hand with increased concentration and increased multiplicity and variety of con- nections. Autor of the post: Undefined They are usually distinguished Post Date: Wed, 13 Aug 2008 13:59:24 +0000 Carrying with us this general conception, let us now study its structure more closely : considering, at first, not any particular forms of it but its universal form. 9 The nervous system is composed of two tissues, which both differ considerably from those composing the rest of the organism. They are usually distinguished from one another by their colours as grey and white, and by their minute struc- tures as vesicular and fibrous. Autor of the post: Undefined Let us see what we Post Date: Wed, 13 Aug 2008 13:48:47 +0000 Chemical analyses have not at present thrown more than a nickering light on the consti- tution of nerve-matter in general, or on the constitution of one kind of nerve-matter as contrasted with the other. All that can be asserted with safety is, that each kind contains phosphatic fats and protein-substances; but that these com- ponents are both differently distributed and in different states in the two tissues. Let us see what we are told about them by the microscope, aided by chemical re-agents. 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