(In the vertebrate eye, the spot where the optic nerve pierces the external layer of the retina is not sensitive to light impressions; hence, it is called the "blind spot.")
When this mollusk sees periophthalmus bounding over the sands (and that it does see is beyond all question), what does it do? It contracts a thousand or so of little bladder-like cells in the skin of its back, thereby discharging a hailstorm of minute concretions in the face of its enemy. The fish, terrified and amazed by the volley, often turns aside, and the mollusk is saved. Thus we see that its dorsal eyes are of great service to onchidium.
The Greeks were, unwittingly, very near an anatomical truth when they ascribed to certain monsters, called cyclopes, only one eye apiece, which was placed in the centre of their foreheads. The cyclopean eye exists to-day in the brains of men in a rudimentary form, for in the pineal gland we find the last vestiges of that which was once a third eye, and which looked out into the world, if not from the centre of the forehead, at least from very near that point. There is alive to-day a little creature which would put to shame the one-eyed arrogance and pride of Polyphemus, and Arges, and Brontes, and Steropes, and all the rest of the single-eyed gentry who, in the days of myths and myth-makers, inhabited the "fair Sicilian Isle." The animal in question is a small lizard, called Calotis. Its well-developed third eye is situated in the top of its head, and can be easily seen through the modified and transparent scale which serves it as a cornea. Many other lacertilians have this third eye, though it is not so highly organized as it is in the species just mentioned. A tree lizard, which is to be found in the mountains of East Tennessee and Kentucky, has its third eye quite well developed. This little animal is called the "singing scorpion" by the mountaineers (by the way, all lizards are scorpions to these people), and is a most interesting creature. I heard its plaintive "peep, peep, peep," on Chilhowee Mountain a number of times before I became aware of the fact that a lizard was the singer. On dissection, the third eye will be found lying immediately beneath the skin; it has a lens, retina, and optic nerve.
Thus we see that the sense of sight is to be found in animals very low in the scale of life. From a simple acc.u.mulation of pigment-cells which serves to arrest light rays (in simple organisms such as rotifers) to that complex and beautiful structure--the human eye--the organs of vision have been developed, step by step.
We will also see in the course of this discussion that, just as these simple and primal organisms have given place to more complex forms, just so have the operations of mind become higher and more involved. We see, in periopthalmus, a creature exceedingly well adapted by form, function, and intelligence to its manner of life. We must admit, in fact, the correlation and interdependence of morphology, physiology, and psychology in the evolution of this creature from its ancestral form to its present status.
The primitive organ of audition as it is to be observed in creatures of simple, comparatively speaking, organization is as simple as is the anatomy of the animals in which it is found. Commonly, it is a hollow hair, which is connected by a minute nerve-filament with the sensorium.
Sound vibrations set the hair to vibrating, which in turn conveys the vibrations to the nerve-filament, and so on to the auditory centre.
Sometimes the hair is not hollow; in this case, the root of the hair is intimately a.s.sociated with nerve-filaments which take up vibrations.
It is highly probable that the majority of the lower animals, especially those which are sound-producers, can hear just as we hear. It is also highly probable that the so-called deaf animals can hear, just as we hear when we have either been born deaf, or through disease have lost the power of hearing--by _feeling_ the sound waves.
Owing to our own lack of acuteness, all of the problems involved in this question of audition in the lower animals will, probably, never be definitely settled; yet, reasoning by a.n.a.logy, we can, approximately, solve some of them.
By far the larger number of entomologists locate the auditory organs of insects in their antennae. I have only to mention the names of such men as Kirby, Spence, Burmeister, Hicks, Wolff, Newport, Oken, Strauss, Durkheim, and Carus, who advance this opinion, to show what a formidable array of talent maintains it. Yet my observations lead me to believe otherwise, though these authorities are in part correct. As far as Lepidoptera are concerned, and certain of Hemiptera, they are right--the antennae in these creatures are the seat of the organs of audition. But in Orthoptera, in most of Coleoptera, Hymenoptera, and Diptera, and in certain bugs (Hemiptera), they are located elsewhere. The habit that almost all insects have of retracting their antennae when alarmed by noise, or otherwise, has done much to advance and strengthen the opinion that these appendages are the seat of insect ears; yet I am confident that in nine cases out of ten the antennae are retracted through fear of injury to them, and not through any impression made on them by sound.
The antennae are the most exposed and least protected of any of the appendages or members of the insect body; hence their retraction by insects when alarmed is an instinctively protective action. They shelter them as much as possible in order to keep them from being injured.
Again, although the antennae of most insects are provided with numerous sensitive hairs, or setae, we have no right to a.s.sume that these hairs are auditory; no "auditory rods," otoliths, etc., are to be found generally in antennae, yet there are exceptional instances. Leydig found auditory rods in the antennae of _Dyticus marginalis_ (Furneaux[17]), the giant water-beetle, and I myself have observed them in _Corydalis cornuta_ and other neuropterous insects. I am inclined to believe that the entire order of Neuroptera has antennal ears, and should therefore in this respect be cla.s.sed with Lepidoptera.
[17] Consult Furneaux, _Life in Ponds and Streams_, p. 325.
In gra.s.shoppers and crickets the ears are situated in the anterior pairs of legs. If the tibia of a gra.s.shopper"s anterior leg be examined, two (one before and one behind) shining, oval, membranous disks, surrounded by a marginal ridge, will be at once observed. These are the tympana or ear-drums of the ear of that leg. Where the trachea, or air-tube, enters the tibia it becomes enlarged and divides into two channels; these two channels unite again lower down in the shaft of the tibia. The tracheae of non-stridulating grylli are much smaller than those of sound-producing gra.s.shoppers. The same may be said of the tibial air-tubes of the so-called dumb crickets. In gra.s.shoppers and crickets the ear-drums lie bathed in air on both sides--the open air on the external side and the air of the air-tube, or trachea, on the inside.
Lubbock calls attention to the fact that "the trachea acts like the Eustachian tube in our own ear; it maintains an equilibrium of pressure on each side of the tympanum, and enables it freely to transmit atmospheric vibrations."
In gra.s.shoppers the auditory nerve, after entering the tibia, divides into two branches, one forming the supratympa.n.a.l ganglion, the other descending to the tympanum and forming a ganglion known as Siebold"s organ. This last-mentioned ganglion is strikingly like the organ of Corti in our own ear, and undoubtedly serves a like purpose in the phenomenon of audition.
The organ of Corti is composed of some four thousand delicate vesicles, graduated in size, each one of which vibrates in unison with some particular number of sound vibrations. The organ of Siebold in the gra.s.shopper"s ear begins with vesicles, of which a few of the first are nearly equal in size; these vesicles then regularly diminish in size to the end of the series. Each of these vesicles contains an auditory rod, and is in communication with the auditory nerve through a delicate nerve-fibril. I have observed that each of these nerve-fibrils swells into a minute ganglion immediately after leaving its particular vesicle; the function of these ganglia is, I take it, to strengthen and reenforce nerve-energy. No other observer mentions these ganglia, as far as I have been able to determine; they may have been absent, however, in the specimens studied by others, yet in the specimens studied by myself--the "red-legged locust" (_Melanoplus femur-rubrum_, Comstock)[18] and the "meadow gra.s.shopper" (_Xiphidium_), they were always present.
[18] Consult Comstock, _Manual for the Study of Insects_, p. 110.
That gra.s.shoppers, locusts, and crickets can hear, no one who has observed these creatures during the mating season will for one instant deny; they hear readily and well, for in most of them the sense of hearing is remarkably acute.
Immediately behind the wings of flies two curious k.n.o.bbed organs are to be observed; these are considered to be rudimentary hinder wings by entomologists, and are called the halteres. Bolles Lee and others of the French scientists call them _balanciers_. This latter name I consider the correct one, for these organs unquestionably preside over alate equilibrium: they are true balancers. I do not propose to enter into any discussion as to whether these organs are rudimentary wings or not; suffice it to say that they appear to me to be organs fully developed and amply sufficient to serve the purposes for which they were created. Whether or not in the process of evolution there has occurred a change of function, is a point which will not be discussed in this paper. As they now exist, I deem them to be auditory organs of Diptera (flies, gnats, etc.).
The semicircular ca.n.a.ls are, to a great extent if not entirely, the seat of equilibration in man. Any derangement or disease of these ca.n.a.ls interferes with equilibration; this is well shown in Meniere"s disease, in which there is always marked disturbance of the equilibrating function.
If the balancers of a horsefly be removed, the insect at once loses its equilibrium; it cannot direct its flight, but plunges headlong to the ground. The same can be said of _Chrysops niger_--in fact, of the entire family of Tabanidae, of the gall gnat (_Diplosis resinicola_, Comstock), and of the March flies (_Bibionidae_). These widely differing flies const.i.tute the material from which I have derived my data; I will venture to a.s.sert, however, without fear of contradiction, that what has been said about the flies mentioned above is equally true of all flies.
When the k.n.o.bbed end of the balancers of the horsefly (_Taba.n.u.s atratus_, Comstock)[19] are examined with the microscope, the cuticle will be found to be set with minute hairs or setae; some of these hairs penetrate both cuticle and hypoderm, are hollow, and receive into their hollows delicate nerve-fibrils. These nerve-fibrils pa.s.s inward toward the centre, and enter ganglia, which in turn are in immediate connection with the great nerves of the balancers. There is but one nerve in the insect"s body that is larger than the balancer nerve, and that is the optic nerve; hence, it is natural to infer that the balancer nerve leads to some special sense centre. This centre in my opinion is, unquestionably, the seat of the auditory function.
[19] Consult Comstock, _loc. cit. ante_, p. 455.
It has been demonstrated beyond doubt that a.n.a.logous hollow hairs, or setae, are prominent factors of audition in many animals, notably crustaceans, such as the lobster, the crab, and the crayfish, and many of the insect family; hence, it is logically correct to conclude that the hollow hairs on the balancers of flies are likewise auditory hairs.
Moreover, there are grouped about the bases of these k.n.o.bbed organs certain rows of vesicles, which contain auditory rods almost identical in appearance with the auditory rods of the gra.s.shopper. Indeed, I have found those in the upper row of vesicles to be precisely similar in appearance to the rods found in Melanoplus.
I have determined that in the horsefly (_Taba.n.u.s atratus_) there are six rows of these vesicles, and that they are graduated in size. There are in the k.n.o.bs of the balancers minute spiracles (I do not think that these have been pointed out before by any other observer) through which air pa.s.ses into the large, vesicular cells which make up the greater portion of the k.n.o.bs; spiracles are also to be found in the shafts of the balancers, thus providing an abundance of air to the internal structures of these organs and allowing for the free transmission of sound vibrations.
I am well aware of the fact that in considering these organs to be the ears of flies, I antagonize Lee and others who consider them olfactory in character.[20] The position I take in regard to these organs is, however, a tenable one, and one that cannot easily be overthrown.
[20] Bolles Lee, _Les Balanciers des Dipteres_; quoted also by Lubbock, _Senses, Instincts_, etc., pp. 110, 111.
The ears of Lepidoptera (b.u.t.terflies) are situated in their antennae. This fact has been clearly demonstrated by Lubbock, Graber, Leydig, and Wolff.
Newport has made an especially exhaustive study of the antennae of insects; and he, too, places the organs of audition in these appendages.[21] But in Coleoptera my experiments and microscopical researches compel me to a.s.sert that I differ somewhat from the conclusions of the above-mentioned authorities. These gentlemen locate the ears of beetles also in their antennae. Lubbock bases his conclusions on an experiment of Will--an experiment which, if it had been carried a little further, would have demonstrated the fact that the ears of beetles are not in their antennae, but are, on the contrary, in their maxillary palpi.
[21] Newport, _The Antennae of Insects_, Entomol. Society, Vol. II.
Will put a female Cerambyx beetle into a box, which he placed on a table; he then put a male Cerambyx on the table, some four inches from the box. When he touched the female she began to chirrup, whereupon the male turned his antennae toward the box, "as if to determine from which direction the sound came, and then marched straight toward the female."
Will concluded from this that the ears of the beetle were located in its antennae.[22]
[22] Will, _Das Geschmacksorgen der Insecten_, Wiss. Zool.; quoted also by Lubbock, _Senses, Instincts_, etc., p. 96.
Seeing that Will"s experiment as described by him was incomplete, I took a pair of beetles belonging to the same family (genus _Prionus_), and determined the true location of their ears by a system of rigid exclusion. These beetles, when irritated, make a squeaking chirrup by rubbing together the prothorax and mesothorax.
When I irritated the female she began to chirrup, and the male immediately turned toward the small paper box in which she was confined.
I then removed the antennae of the male, and again made the female stridulate; the male heard her, and at once crawled toward her, although his antennae were entirely removed.
This showed conclusively that the organs of audition were not located in the antennae, as Will supposed and as Lubbock advocates. I then removed the maxillary palpi of the male, after which the insect remained deaf to all sounds emanating from the female.
Again, I took an unmutilated male, which at once turned and crawled toward the chirruping female. I then removed its l.a.b.i.al palpi, leaving maxillary palpi and antennae intact; it heard the female and made toward her. The maxillary palpi were then removed (the antennae being left _in situ_), and at once the creature became deaf.
If the maxillary palpi of long-horned beetles be examined, certain vesicular organs, each containing a microscopic hair, will be observed in the basal segments; these, I take it, are auditory vesicles. In some of the Coleoptera I have found auditory rods in the apical segments, though this is by no means a common occurrence. In Cicindelidae and Carabidae these auditory vesicles are exceedingly small, and require a very high-power objective in order to be clearly seen.
In justice to other observers I must say, however, that I am inclined to believe that in all beetles the antennae in some way aid or a.s.sist audition, but they are adjuncts, as it were, and not absolutely necessary. It is a matter of easy demonstration to show that some of these insects hear less acutely where they are deprived of their antennae.
I presume they are about as necessary in audition as are the external appendages of the human ear; this, however, is mere supposition, and has no scientific warrant for its verity.
I have purposely said but very little about the senses of touch, taste, and smell in this discussion of the senses in the lower animals. These three senses have been so exhaustively treated by Lubbock in his _Senses, Instincts, and Intelligences of Animals_, that I could not hope to introduce any new data in regard to them. Graber, Frey, Leuckart, Farre, Hertwig, and a host of others have likewise investigated these senses most thoroughly.
As to the senses of sight and hearing, the matter presented a different aspect. I was confident that I could add somewhat to the knowledge already formulated, consequently I have treated these senses at some length. Technicalities and the details of microscopic investigation, especially microscopic anatomy, have been omitted; they have no place in a work like this.
CHAPTER II
CONSCIOUS DETERMINATION
Conscious determination, or, effort induced by conscious volition, is the basic mental operation upon which is reared that complex psychical structure which is to be found in the higher animals, and especially in man--the highest product of evolutionary development.
By conscious volition is not meant that consciousness which appertains to the child of two or three years, who, at that age, recognizes the _ego_. Ego-knowledge, while undoubtedly present in some of the higher animals, such as the dog, monkey, horse, cat, etc., is not a factor in the psychical make-up of any of the lower animals (insects, crustaceans, mollusks, etc.). But consciousness, so far as volition or choice is concerned, enters into the _psychos_ of animals exceedingly low in the scale of animal life.
We have seen in the chapter on the senses in the lower animals, that animals possess one or all of the five senses--touch, taste, smell, sight, and hearing; we will see in a later chapter that some of them likewise possess certain other senses which man has lost in the process of evolution.
Now, let us very briefly discuss the _modus operandi_ through which and by which conscious determination and other psychical manifestations arise from the physical basis--the senses.[23] I have a.s.serted, and, as I believe, I have demonstrated elsewhere, the interdependence and correlation of physiology and psychology. Furthermore, I wish to be plainly understood as also a.s.serting the physical basis and origin of all psychical operations whatever they may be.
[23] "Sensorial impression is at the bottom of all our ideas, all our conceptions, though it may at first conceal itself in the form of a binary, ternary, quaternary compound; and, on our methodically pursuing the inquiry, it is easily recognizable--just as a simple substance in organic chemistry may be always summoned to appear, if we sit down with the resolution to disengage it from all the artificial combinations which hold it imprisoned."--LUYS, _The Brain and its Functions_, p. 252.
Mind is always a.s.sociated, according to our experience and knowledge (and this question must be studied objectively) with a peculiar tissue which is only to be found in animal organisms. This tissue is called nerve, and is made up of cells and, broadly speaking, prolongations of cells which are called nerve-fibres.