INthe stimuli and the responses they produce are relatively few, but in the case of higher animals the different stimuli are very numerous in comparison, and the responses so varied that we cannot predict with any certainty what an animal will do in response to a particular stimulus in particular circumstances. Moreover, the responses are usually rapidly made, not requiring at least several hours as, with few exceptions, in the case of plants. This is due to the possession of ’sense ’cells and a nervous system by which the immediate effect of a stimulus upon sense cells can be very rapidly conveyed to all other parts.
Sense or Sensory Cells are those cells which are affected by stimuli —such are those in the eye, which are affected by light, and those in the ear, affected by sound. Since they receive stimuli they are also known as receptors. They are of two kinds : exteroceptors on the surface, or near it, which give us information about our surroundings, e.g. those quoted above, and proprioceptors situated internally, which inform us of our internal state, e.g. those producing the sensations of physical tiredness and of stomach-ache. We are not concerned with the structure of sense cells, as such details lie outside the scope of this work ; it being sufficient to say that they are extremely small and delicate, many kinds of exteroceptors being characterized by the possession of fine threads protruding above the general surface. In function they vary tremendously, for each kind of receptor is affected, as a general rule, by only one kind of stimulus and, in some cases, only within narrow limits, e.g. there is a very wide range of electro-magnetic waves : some affect the surface of the body, producing the sensation termed warmth—these are radiant heat waves ; others affect only the receptors in the eye—light waves : and yet others, when we touch a source of such waves, give us an electric shock—electric waves.
Receptors, therefore, are in function the most highly specialized cells of the body. They are classified according to the information they give us. The skin and the inside of the mouth, as well as the tongue, possess several different kinds of receptors, so that we can tell by touch what the general shape of an object is, and whether it is hot or cold, rough or smooth, hard or soft, wet or dry. Pain may be produced as a result of excessive stimulation, e.g. excessive pressure such as pinching, and a temperature high enough to burn. All other exteroceptors are gathered together in special places : those sensitive to taste on the surface of the tongue, to smell in the olfactory organs in the nose, to light in the back of each eye, to sound in the ears. These places are the sense organs, and most of them have some acoessory parts by which they are able to function more efficiently.
The proprioceptors are mainly to be found in the muscles. Those in the muscles of the trunk and limbs give us the sensation of physical well-being or tiredness, whilst those in the muscles of the gut wall are sensitive to pressure. The gut lining has proprioceptors which produce the sensations of hunger and thirst. This by no means exhausts the list, but the remainder are best dealt with when reflex action is understood.
The Nervous System
In order to understand animal irritability it is necessary to deal with the units which make up the nervous system. These units, single nerve cells or neurons, are all similar, each possessing a long fibre which branches out into dendrons at its end, the dendrons in turn branching into extremely fine processes. A nucleus is situated somewhere in the cell, which has other shorter branched processes in addition to the axon. These are called the receptive dendrites, while those of the axon are known as the terminal dendrites. The whole nervous system consists of a brain and spinal cord, ganglia and nerves.
The commonest arrangement of neurons is that known as a reflex are where three neurons are involved, one having its receptive dendrites in sensory cells and its terminal dendrites in the brain or spinal cord in which the second neuron lies, whilst the third has its receptive dendrites in the central nervous system and the terminal dendrites in a structure which works and which is therefore termed a motor organ, being either a muscle or gland. When active, neurons transmit what are called impulses, the activity being accompanied by extremely slight electrical, chemical and temperature changes, and proceeding from receptive to terminal dendrites in every case. In man the speed of impulses is about 30 metres per second or 70 miles per hour.
The whole are works as follows : the receptors are affected by a stimulus, causing them to set up impulses which pass to the central nervous system along a sensory neuron. The impulses pass in sucoession along the receptive dendrites and dendrons, through the cell to the axon, and finally along the terminal dendrons and dendrites. They now jump gaps between sensory and relay neurons, and in the latter pass in sucoession along its receptive dendrites and dendrons, axon, terminal dendrons and dendrites to reach the motor neuron. This in its turn transmits the impulses to a motor organ. Then and only then docs the motor organ work. If it is a muscle it contracts ; if a gland then it produces its particular secretion. Moreover, it only works whilst it is receiving the impulses. Such an action of the motor organ is quite automatic ; in many cases the animal knows nothing about it, so that it is unconscious action.
Examples of such purely unconscious action are the beating of the heart and breathing during sleep, the churning of food by the stomach walls, the secretion of bile from the liver and of pancreatic juice by the pancreas when there is food in the duodenum.
Reflex action which we know about occurs when we are blinking, sneezing, dropping a hot object or doing a knee jerk. In the great majority of cases the action occurs near the place where the stimulus acted—it is for this reason that it is termed reflex action, for the impulses are reflected or turned back along their original path in the nerve in which they travel, but they do not travel in the same neurons on the inward and outward journeys since neurons are either afferent or efferent in function. upon a drawing-pin, as a result of which we jump up, unintentionally call out ’Oh ! ’, feel for the offending article and perhaps look round for the originator of the little ’joke ’ —all actions done without thinking, since we ’find ourselves doing them. ’
It appears to be a rule that in vertebrates impulses shall traverse three neurons before reaching a destination. The usual state of affairs is as shown in the spinal cord. It should be noticed that all simple reflex actions occurring in the spinal cord reach the brain via an ascending fibre, while the motor neurons are also connected with the brain by descending fibres.
All nerves except those in the head have a connection with the spinal cord, each nerve splitting into a dorsaland a ventral just before doing so. Some are connected with ganglia which lie outside and below the vertebral column. These go to all the organs lying in the body cavity, and with their ganglia form the sympathetic nervous system. All the other nerves connected with the spinal cord are spinal nerves, made up of both afferent and efferent axons, so that each is a mixed nerve. The axons of afferent neurons pass into the spinal cord by way of the dorsal root, their nuclei being situated in a ganglion on it. Axons of all the efferent neurons pass outwards by way of the ventral root, either back along the spinal nerve or ventrally to a sympathetic ganglion. The area occupied by the nerve endings in the spinal cord is grey in comparison with the rest and is therefore termed grey matter, the rest being called white matter. This white matter consists largely of axons which run along the spinal cord and, functionally, form six sets, those carrying impulses from any particular part of the cord : ascending ; descending from spinal cord ; descending from brain ; and those carrying impulses conversely to these three.
Since the terminal dendrites of any afferent neuron are interlaced at the synapse with the receptive dendrites of several efferent neurons, it becomes possible for impulses from all sources to get to all the destinations possible. The whole arrangement may be likened to a complex telephone system—all callers being equivalent to receptors ; the ’lines ’to axons ; the automatic exchanges to the central nervous system ; minor exchanges to the ganglia ; and those who receive a call, to the motor organs.
Examples of Reflex Action.
The Knee Jerk
This is best demonstrated by pupils working in pairs. One sits with his or her legs crossed lightly at the knee with the lower leg vertical. The other taps the upper knee lightly at the base of the knee-cap. A slight kick of the leg occurs.
The Pupil Reflex
This should be done in a darkened room in front of a mirror. A lighted candle is held at arm ’s length at the side and brought gradually nearer the eye nearest the candle. If the pupil of the eye is observed in the mirror it will be seen to get smaller and smaller. The difference in size of the pupil of a cat in light and darkness is well known.
The Swallowing Reflex
The larynx should be lightly held between the thumb and finger. On swallowing, it will be felt to move upwards and forwards. A scries of swallowing movements is then attempted, but after the first two or three attempts no movement of the larynx can be detected. At the back of the pharynx is a sensitive area which is stimulated by the food or fluid which is swallowed and sets in operation the reflex action which closes the glottis and forces the food down the oesophagus by peristalsis. When repeated attempts at swallowing are made with only saliva in the mouth, the supply of this soon becomes inadequate to induce the reflex action to occur. It should be noted that the first part of the act of swallowing is voluntary and is produced by the action of the tongue in forcing the food or liquid back into the pharynx assisted by the muscles of the cheek. That the remainder of the act is reflex is evident from the unsucoessful attempts made to ’recall ’a cherry stone or orangewhich has slipped back too far and has passed into the pharynx, which everyone has experienced at some time or other.
The Scratch Reflex
If the flanks of a dog are tickled, scratching reflex movements are induced. They can often be induced in a cat by stroking it under the outer angle of the jaw. Lightly tickling the ear of a sleeping dog or cat is followed by a reflex ’flip ’of the ear to remove the ’fly, ’ and if persisted in will often awaken the animal and induce the scratch reflex.
Reflex actions in lower animals are very numerous. Well-known examples are the withdrawal of the horns of a snail on their being touched and the flight of moths and other insects towards the light, due in this case to unequal illumination of the eyes. The reflex response of the anterior part of an earthworm to a bright light can be shown by inducing a worm to crawl into a glass tube covered with a dark cloth. If the front end of the worm is uncovered near an electric light, an immediate withdrawal of the anterior end into the shaded part of the tube will occur.
The reflex responses of Amoeba, Paramecium and Hydra are given on pp. 395 to 396.