Book - Vertebrate Zoology (1928) 32
Vertebrate Zoology G. R. De Beer (1928)
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Chapter XXXII The Sense-Organs
The Eye
With regard to the eyes, two points of interest present themselves. The first concerns the method of accommodation of the eye for seeing objects at different distances, and the second relates to the capacity of some animals to see a single object with both eyes at the same time.
Accommodation is a simple optical problem concerning the focal length of the lens, the distance of the viewed object, and the distance between the lens and the retina. These three terms must be in relation according to the laws of optics if there is to be a clear image of the object on the retina. The first and the third term are within the animal, and are there- fore variable, while the second, the distance of the object, is obviously external to the animal and not under its direct control. It is found that some animals accommodate by alter- ing the distance between the lens and the retina, and others by altering the focal length of the lens itself.
Cyclostomes and Selachians may be left out of account, for their eyes can accommodate but little if at all. In the bony fish, the eye when at rest is accommodated for near vision. This fact is in relation to the optical nature of the medium in which they live, water, through which it is not possible to see very far. The lens is attached to the eye-cup by a retractor lentis muscle, and when this contracts, the lens is brought nearer to the retina, and the eye can then focus objects which are farther away. Land-vertebrates always have their eyes focussed at rest for distant vision, which enables them the earlier to see their prey or their enemies. So, in amphibia, the lens is attached to the eye-cup by a protractor lentis muscle.
By its contraction, the distance between the lens and the retina is increased, and the eye can then focus near objects.
In all the cases so far mentioned, the lens is a rigid body with a fixed and definite focal length, and which has to be moved bodily in order to accommodate the eye. In the remaining vertebrates, the lens is elastic and capable of varying its convexity and focal length. In reptiles, accommodation for near vision is brought about by contraction of the circular muscle of the iris, which has as its effect the increase in con- vexity of the lens, which thus tends to become spherical. In the birds, there is in addition a striated muscle called Crampton's muscle, contraction of which decreases the diameter of the eyeball in the neighbourhood of the junction between the cornea and the sclerotic. This causes the surface of the cornea to become more convex, and assists the lens to bring rays of light from near objects to a focus on the retina.
The method of accommodation in the mammals differs from that in other vertebrates. The lens is suspended by the suspensory ligament, which is kept tense by the elasticity of the lens trying to revert to the spherical shape. The suspensory ligament is attached to the ciliary process. The ciliary muscle is attached to the cornea in front and to the choroid behind, so that when it contracts, the choroid and ciliary process are brought forwards. This forwards movement of the ciliary process reduces the tension on the suspensory ligament, and the lens is allowed to become more spherical, which increases its refractive power and enables it to accommodate the eye to near objects. The change in focal length of the lens is there- fore only indirectly due to the action of the ciliary muscle.
In some vertebrates, and especially those of nocturnal habits, the eyes do not accommodate for distance at all, which fact does not prevent them from enjoying good sight, as does the owl. In daylight, the pupil may be so contracted as to simulate a " pinhole " camera, in which accommodation is unnecessary.
In mammals the ciliary muscle is contracted by impulses passing in fibres of the parasympathetic system through the oculomotor nerve and the ciliary ganglion. Other fibres following the same path constrict the pupil (contract the sphincter and relax the radial muscles of the iris). The pupil is dilated by impulses in fibres coming from the sympathetic system of the neck.
In the lower vertebrates, the eyes are on each side of the head, and there is little, if any, overlap in the two fields of vision. In these forms, the decussation or crossing- over of the fibres at the optic chiasma is complete : the fibres from an eye run to the opposite side of the brain. In the higher vertebrates, on the other hand, it is common for the fields of vision of the two eyes to overlap considerably, and even to coincide. In these cases both eyes can be brought to bear on a single object, which enables the animal to estimate distance. This is of importance in arboreal animals which have to gauge the strength of their efforts in leaping from branch to branch. This binocular vision is present in the monkeys and man, in the owls, and to a varying extent in other animals.
The possession of binocular vision is a great advantage, but it robs the animal of vision over a large radius around it, which it would have if its eyes diverged widely on each side of the head. It is found as a rule that the more timid mammals have widely divergent axes of vision, amounting to nearly two right angles in the case of the rabbit. The rabbit therefore can see objects almost everywhere all round it ; it uses its eyes qualitatively to warn it of the approach of enemies. The axes of vision of the lion, on the other hand, are almost parallel ; it sacrifices a large field of vision for the advantage of using its eyes quantitatively in estimating distance and spatial relations.
In mammals with binocular vision, it is important that the movements of the two eyes should be co-ordinated so that their axes of vision remain more or less parallel with one another. In other animals each eye can be moved separately, and this faculty is extremely developed in Chamaeleo.
The fibres from the eyes of mammals such as the rabbit decussate almost completely at the optic chiasma. In the monkeys and man, on the other hand, the decussation of the fibres is incomplete. Fibres from the lateral portion of the retina of each eye do not cross-over, but go to the same side of the brain. It is the fibres from the median portions of the retinae which cross-over and go to the opposite side of the brain. The images of one object fall on corresponding points in the two retinae, and the fibres from these corresponding points run to one and the same side of the brain.
Many of the lower vertebrates have been shown to be sensitive to different colours. It is supposed that the cones of the retina are sensitive to colour, and that the rods only perceive light and dark. Nocturnal animals such as owls and bats have scarcely any cones, and are presumably colour-blind. The proportion of cones in the retina increases as a rule from the lower to the higher vertebrates. In the higher Primates and in man, the eyes have " corresponding points " of optimum sensitiveness (the macula lutea or " yellow spot "), in which the retina consists of cones only, without any rods.
In some vertebrates the eyes have been lost. They are very degenerate in some of the Cyclostomes, which lead a semi-parasitic life, and in the Urodele Proteus, which inhabits the dark caves of Carniola. Fish which live in the dark of the abyss of the ocean or in caves may be blind and eyeless, as, for example, Ipnops, Amblyopsis, and Lucifuga. Among mammals, the eyes are often reduced in forms which live in the dark in burrows underground. The common mole is an example, and a comparable but even more far-reaching reduc- tion of the eyes has taken place independently in the " marsupial mole " Notoryctes.
The Pineal
There is no doubt that the early vertebrates were capable of seeing by means of their pineal organs, through the pineal foramen in the roof of the skull, though possibly not of forming an image. Among living forms, Petromyzon has two pineal organs ; other forms have only one, which may represent the original right or left organ. The pineal is least degenerate in Sphenodon. It is in the form of a vesicle of which the upper wall forms the lens and the lower the retina, which is connected by nerve-fibres with the brain. This retina is not " inverted," as is that of the paired eyes. Sur- rounding the retina is pigment, and the organ is sensitive to light.
In birds and mammals there is no pineal foramen in the skull, and the pineal organ remains beneath the bone. It is reduced to a solid vestige and its function is changed from that of a visual organ to an organ of internal secretion, or ductless gland.
The Ear
The most primitive part of the ear is the utricular portion with its semicircular canals and ampullae. Myxine has one, and Petromyzon has two semicircular canals on each side. All other Craniates have three, in planes at right angles to each other. In the ampullae are the statolithic particles which are supported on sensory cilia. Gravity makes these particles weigh on the cilia immediately beneath them, whatever the position of the animal, and so the animal is informed of its position with regard to the vertical according as to which of the cilia are so stimulated. The semicircular canals contain fluid, the endolymph, as do all parts of the auditory sac. When the animal starts or ceases moving, a flow of endolymph takes place in the semicircular canals, which resolve the direction of the movement into resultants in the three planes of space in which they lie. While the statoliths are static, the semicircular canals are dynamic organs of balance.
Hearing is the perception of mechanical vibrations of low frequency. Fish are capable of hearing with their auditory organs, but this sense only becomes important in the verte- brates which have left the water, and are therefore subject to vibrations in air. This is significant because these animals are also the first to emit vocal sounds. Since these animals are autostylic and no longer breathe by gills, the spiracular cleft and the hyomandibula are no longer needed to subserve their primitive functions ; they give rise to the tympanic cavity (and Eustachian tube) and columella auris (stapes) respectively. The vibrations of air impinge on the tympanic membrane or ear-drum, and are conveyed by the columella auris across the tympanic cavity to the auditory capsule. The wall of the auditory capsule is imperforate in the fish and in the most primitive Stegocephalia (Eogyrinus). In the remaining vertebrates the auditory capsule has two openings in its wall. One of these is the fenestra ovalis which enables the vibrations to be imparted to the fluid (perilymph) which bathes the auditory sac. The other is the fenestra rotunda ; it is covered by a membrane which absorbs the vibrations in the perilymph and so brings them to an end.
That part of the auditory sac which is actually concerned with hearing is the cochlea, rudimentary in amphibia but well developed in the higher vertebrates. The vibrations of the perilymph are imparted to the endolymph within the cochlea, which in its turn stimulates the sensory cells. In mammals where the development of the ear is at its highest, the auditory ossicles are three in number, the cochlea is long and coiled, and an external ear assists in collecting the air vibrations.
Cyclostomes, fish, and larval amphibia possess a system of sense-organs known as the lateral-line organs, and which serve to appreciate vibrations in water of low frequency. The ear itself is to be regarded as a specialised organ of the lateral-line (or " neuromast ") system.
The Nose
The olfactory organ or nose contains an epithelium which is sensitive to very minute quantities of chemical substances, dissolved or suspended in water, or suspended in air. In Dipnoi and Tetrapods the nose has an open connexion with the mouth cavity, and so enters into the service of the respiratory system, enabling air to reach the lungs without opening the mouth. This connexion does not exist in forms below the Dipnoi (except in Myxine, where the hypophysial sac opens into the gut).
Taste-organs
The nose is a distance-receptor, appreciat- ing chemical substances from afar. Taste-organs, on the other hand, serve for appreciating substances in contact with the animal, and especially in connexion with the opening of the alimentary canal. Taste is a visceral sense, while smell is a somatic sense. While in most vertebrates the taste-organs are restricted to the mouth, in some fish, such as the catfish, they are distributed over the surface of the body.
Jacobson's Organ
Associated with the nose in land- vertebrates is a pair of pouches which constitute Jacobson's, or the vomero-nasal organs. Their function is doubtful, but it is probably concerned with smelling the food in the mouth, with which they are in communication. In some forms, in- cluding man, Jacobson's organs disappear. In the snakes they are very highly developed, and the tips of the forked tongue enter their openings in the roof of the mouth. Sub- stances gathered on the tongue when protruded are thus placed in contact with the sense-organ.
Literature
von Buddenbrock, W. Grundriss der vergleichenden Physiologic. Vol. i.
Borntraeger, Berlin, 1924. Herrick, C. Judson. An Introduction to Neurology. Saunders Co., Philadelphia and London, 1922.
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- Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology
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