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ANATOMY

vertebres,” La Cellule, ix. 1893 ; Retzius, Biologische Untersuch. 1893, and Lenhossek, Anat. Anzeiger, 1893.) Embryologically, each sense-organ is a specialized area of dermal epithelium (except perhaps in the case of the tongue, whose surface is originally hypoblastic). Epithelial cells which line pits or are otherwise placed in positions where they receive an abundant supply of nutrition (gland-cells) are usually highly meta0/ d //s " bolic, and have the power of separating from their substance by some process as yet unknown certain enzymes or other derivative material which they extrude, dissolved in some of the more fluid parts of the cell-content. These secreting cells undergo a physical change during their condition of activity, altering in appearance and reaction; but as yet no intelligible physical explanation has been given of this process, or of the correlation which exists between the specific enzyme produced by each gland and the physiological necessity of the part of the organism in which the gland lies. Sometimes single gland-cells occur in a continuous epithelium, as, for example, the mucus-secreting goblet-cells on the villi of the small intestines; but, generally, gland-cells are arranged at the bottom of tubular pits for the convenience of nutrition and of the accumulation of the secretion. The forms of the several kinds of glands depend on the degree of branching of these gland tubes. (On the structure of glandular epithelium, see Langley, Journ. of Physiol. 1889; and Nicolaides, Centralblatt fur Physiol. 1889 -) Nervous tissue is the recipient of all stimuli, the seat of all sensations, the starting-point of all motor impulses, the regulator of vascular and other activities, tissue* an8 the controller of the processes of nutrition and secretion. It is connected with every organ, and its terminations are distributed in almost every tissue of the body. It is not to be wondered at, therefore, that its structure is of amazing complexity, or that our knowledge of it is as yet imperfect and fragmentary. The morphological conception of the organization of nervous tissue has undergone fundamental modifications in recent years, as the structure of its component form-elements has become better understood. The methods of staining introduced by Golgi and Weigert have rendered it evident that these elements are distinct from each other, and that their communications are indirect. The unit of structure which is called a neurone (Fig. 4) consists of three parts:—(1) A cell-body ox perikaryon, containing a nucleus with a complex karyomiton ; around this nucleus are other chromophile elements called Nissl bodies, capable of being stained by methylene blue, while the rest of the perikaryon consists of a twofold achromatic substance, a fibrillar or spongioplasm-element, supposed to be conductive, and a fundamental or trophoplasm-element, supposed to be the seat of metabolic chemical changes. (2) A single filament or axon, starting from the perikaryon by a cone of origin containing no chromatic elements. After a course of variable length this becomes coated with a sheath of myelin, and passes, often far afield, to its ultimate destination, terminating by branching into a short tuft of filaments which may be included in a muscle fibre or other end organ, or else may be in close proximity to, but not in contact with, the second order of processes, about to be described, of another perikaryon. On its way to its terminal tuft an axon may give off collateral filaments, which also may end in one or another of these ways. (3) From the other end of the perikaryon, usually from a part containing chromophile elements, there arises a series of one or more short, repeatedly-branching filaments, called dendrites, which form an arborescent outgrowth from the nerve-cell, and are related to the terminal tufts of the axons of contiguous cells. At the ends of

the dendrites are small enlargements called gemmules, and sometimes sharp-pointed processes called thorns. It has been supposed that the nerve currents in the dendrites are cellipetal and in the axons cellifugal. The motor nerves are all axons of cells, and it has been supposed that the sensory fibres are elongated dendrites. The aggregations of neurones derived from the epiblast of the dorsal groove of the embryo make up the nervous centres. Other peripherally distributed neurones make up ganglia. In different parts of the nervous system the arrangement of the neurones varies in grades of complexity. In the simplest cases the dendrites carry the a

Fxo. 4.—Diagram illustrative of the neurone theory. Each neurone consists of perikaryon (b, c, d, e), axon (/g), and dendrites. A, Simple arc of two neurones, one having its dendrites as senscry recipient fibres in the skin (2), and its axon ending in branches forming a synapsis with the dendrites of the motor cell (c), whose axon ends in the muscle fibre (3). B, Arc in which a ganglionic neurone (d) is interposed between the sensory and the motor neurones. C, Arc in which an additional or associative neurone (e) is interposed between the ganglionic and the motor neurones. D, Arc in which the perikaryon of the sensory cell is in the recipient epithelium of a special sense organ (1) such as the retina. A ganglionic neurone (d) is interposed between this and the motor neurone. E, a similar arc with an associated neurone between the ganglionic neurone and the motor. sensory impulse to the nerve-cell, wherein the wave of nerve force is in some unknown way reinforced, possibly by metabolic change in the trophoblast or the chromatic element, and is discharged as a motor impulse centrifugally along the axon to the muscle in which it ends. Sometimes the circuit consists of two neurones, one receiving and transmitting the sensory stimulus by its axon to the vicinity of the dendrites of the cell whose axon goes td the muscle. Sometimes a third or even a fourth association-system of neurones may be interposed between the tuft of the first axon and the dendrites of the last or motor cell. The method of relationship between the neurone networks is called synapsis; and there is reason to believe that the degree of approximation of the filaments in these is not always constant, but that sometimes the contiguity of some groups of processes may become close, while in other instances they may recede from each