PHYSIOLOGY 15 tiation of structure, and been varied by the intercalation of numerous mechanical contrivances. ffer- In the hydra each ectoderm cell for, broadly speaking, tia- they are all alike serves three chief purposes of the body. (1) It is sensitive, that is, it is thrown into peculiar mole- rm _ cular agitations, with expenditure of energy, when acted upon by external agents. In man and the higher animals certain cells of the original ectoderm of the embryo are differentiated from their fellows (which, losing to a large extent this sensitiveness, remain as a mechanical covering to the body) by a more exquisite development of this power of reaction, and moreover are differentiated from each other in their relative sensitiveness to different agents, so that one set of cells becomes peculiarly susceptible to light, another set to pressure, and the like. Thus the uniform ectoderm of the hydra, uniformly susceptible to all agencies, is replaced by a series of special groups of cells forming the basis of sensory organs, each group being specially sensitive to one agent, and having the nature of its con stituent cells correspondingly modified. (2) In each ecto derm cell of the hydra the agitations primarily induced by the exciting agent become so modified by changes taking place in the cell that the outcome is not always the same. According to processes taking place in the cell, movement of one kind or another, or no movement at all, may result, and such movement as results may take place immediately or at some other time ; it may be at a time so distant that the connexion between the exciting disturbance is lost, and the movement appears to be spon taneous. In man and the higher animals these more complex " neural " processes are carried on, not by the simple sensory cells which receive the primary impression, but by a group of cells set apart for the purpose. These cells constitute a central nervous system, in which a still further division of labour and differentiation of structure takes place, the simple neurotic processes of the hydra, with its dim volition and limited scope of action, being de veloped in a complex manner into processes which range from simple elaboration of the initial additional agitation of the sensory cell into what we speak of as intelligence and thought. (3) Each octoderm cell, by its tail-like prolonga tion, or by its whole body, contributes to the movement of the animal while still carrying on the two other actions just described. In man and the higher animals the material of the sensory cell and of the central nervous cells is too precious to be wasted in movements ; these accordingly are carried out by groups of cells constituting the mus cular tissue, in which both the sensitiveness and the higher neurotic processes of the primitive cell are held in abey ance ; indeed, the latter have almost disappeared in order that the energy of the protoplasm may be more completely directed to producing those changes of form which deter mine the movements of the animal. Further, the separation in space of these three groups of cells or tissues necessitates the introduction of elements whereby the agitations set up in the sensory cell should be communicated to the central nervous cells, where these agitations are further elaborated, as well as of elements whereby the muscular tissue may receive vibrations from the central nervous cells, so that the movements of the body may be determined by these. Hence strands of irritable protoplasm whose energy is not spent in move ment, but wholly given up to the rapid and easy trans mission of molecular vibrations, unite, as sensory nerves, the sensory cells with the central nervous cells, and, as motor nerves, these with the muscles. Lastly, for the adequate carrying out of complex move ments, the contractile cells, elongated into specially con structed fibres and constituting the muscles, are arranged, with inert tissues such as bones, cartilages, tendons, and the like (tissues of mechanical virtues, manufactured by an active protoplasm, but themselves passive, no longer active), into various mechanical contrivances. Similarly the sensory cells, as notably those of the eye and the ear, set apart to be acted upon by special agents, are provided with special mechanisms in order that the agent may act with more complete precision. Thus the sensory cells consti tuting the retina of the eye, in which alone sensory, visual impulses are generated, are provided with an intricate dioptric mechanism, formed partly of inert tissues such as the lens, partly of peculiarly arranged muscular and nervous elements. In this way the simple ectoderm of the hydra is replaced by a complicated system composed of organs, some of them of extremest intricacy. But the whole system may be re duced to two sets of factors. On the one hand there are organs in the old sense of the word, that is, mechanical arrangements, some connected with the muscles and others connected with the sensory cells, organs whose functions have for the most part to be interpreted on mechanical principles, since their most important factors, putting aside intervening muscular and nervous elements, are the inert products of protoplasm doing simple mechanical work. On the other hand there are organs in the later sense of the word, namely, sensory cells differentiated to be sensi tive to special influences, central nervous cells differentiated to carry on the inner nervous work, muscles differentiated to contract, and nerves differentiated to bind together these three other factors. The work of these latter organs is dependent on the nature of their protoplasm ; mechanical arrangements play but little part in them ; and the results of their activity can in no way be explained on simple mechanical principles. Corresponding with this differentiation of the ectoderm Differ- cells runs a somewhat similar differentiation of the endo- entia- derm cells. In the hydra each endoderm cell appears to tlo of receive some of the food bodily into itself and there to ^ T ^ elaborate it into what may be spoken of as prepared nutri tive material. Some of this material the cell retains within itself in order to renew its own protoplasm ; the rest oozes out to the ectoderm cells, the replenishment of whose pro toplasm is thereby effected with a saving of labour. In the higher animals the preparation of food is far more com plicated. The endodermic sheet of the alimentary canal is folded and arranged into organs called glands, with the mechanical advantage that a large amount of surface is secured within a small bulk ; and the constituent endo dermic cells of their glands pour out, or secrete, as is said, divers fluids into the cavity of the canal, so that much pre liminary preparation of digestion of the food takes place before the food really enters the body. Further, these secreting glandular cells are so differentiated as to pour out special juices acting on special constituents of a meal, and the food subjected in turn to the action of these several juices becomes thoroughly prepared for reception into the body. This reception is carried out by other endoderm cells, which in receiving the digested food probably act upon it so as still further to heighten its nutritive value ; and the absorbed food, before it is presented to the mus cular and nervous tissues, for whose use it is largely, though of course not exclusively, intended, is subjected to the action of other cells, such as those forming the lymphatic glands and the liver, in order that it may be still further elaborated, still further prepared for the final conversion into living protoplasm. As in the case of the tissues and organs of ectodermic origin, so also here, the wide separation in space of the masses of differentiated cells constituting tissues necessi tates the introduction of mechanical contrivances for the
carriage of material from place to place. In the simple