The Evolution of Man, V.2, Ernst Haeckel [free ebook reader for pc txt] 📗

essential parts. The further development--the complicated differentiation and composition of the various parts--is a matter of detail.

(FIGURE 2.318. Eye of the chick embryo in longitudinal section (1. from an embryo sixty-five hours old; 2. from a somewhat older embryo; 3. from an embryo four days old). h horny plate, o lens-pit, l lens (in 1. still part of the epidermis, in 2. and 3. separated from it), x thickening of the horny plate at the point where the lens has severed itself, gl corpus vitreum, r retina, u pigment membrane. (From Remak.))

The chief point in this remarkable evolution of the eye is the circumstance that the optic nerve, the retina, and the pigment membrane originate really from a part of the brain--an outgrowth of the intermediate brain--while the lens, the chief refractive body, develops from the outer skin. From the skin--the horny plate--also arises the delicate conjunctiva, which afterwards covers the outer surface of the eyeball. The lachrymal glands are ramified growths from the conjunctiva (Figure 2.286). All these important parts of the eye are products of the outer germinal layer. The remaining parts--the corpus vitreum (with the vascular capsule of the lens), the choroid (with the iris), and the sclerotic (with the cornea)--are formed from the middle germinal layer.

The outer protection of the eye, the eye-lids, are merely folds of the skin, which are formed in the third month of human embryonic life. In the fourth month the upper eye-lid reaches the lower, and the eye remains covered with them until birth. As a rule, they open wide shortly before birth (sometimes only after birth). Our craniote ancestors had a third eye-lid, the nictitating membrane, which was drawn over the eye from its inner angle. It is still found in many of the Selachii and Amniotes. In the apes and man it has degenerated, and there is now only a small relic of it at the inner corner of the eye, the semi-lunar fold, a useless rudimentary organ (

Chapter 1.

5). The apes and man have also lost the Harderian gland that opened under the nictitating membrane; we find this in the rest of the mammals, and the birds, reptiles, and amphibia.

The peculiar embryonic development of the vertebrate eye does not enable us to draw any definite conclusions as to its obscure phylogeny; it is clearly cenogenetic to a great extent, or obscured by the reduction and curtailment of its original features. It is probable that many of the earlier stages of its phylogeny have disappeared without leaving a trace. It can only be said positively that the peculiar ontogeny of the complicated optic apparatus in man follows just the same laws as in all the other Vertebrates. Their eye is a part of the fore brain, which has grown forward towards the skin, not an original cutaneous sense-organ, as in the Invertebrates.

(FIGURE 2.319. Horizontal transverse section of the eye of a human embryo, four weeks old (magnified one hundred times). (From Kolliker.) t lens (the dark wall of which is as thick as the diameter of the central cavity), g corpus vitreum (connected by a stem, g, with the corium), v vascular loop (pressing behind the lens inside the corpus vitreum by means of this stem g), i retina (inner thicker, invaginated layer of the primary optic vesicle), a pigment membrane (outer, thin, non-invaginated layer of same), h space between retina and pigment membrane (remainder of the cavity of the primary optic vesicle).

FIGURE 2.320. The human ear (left ear, seen from the front, natural size), a shell of ear, b external passage, c tympanum, d tympanic cavity, e Eustachian tube, f, g, h the three bones of the ear (f hammer, g anvil, h stirrup), i utricle, k the three semi-circular canals, l the sacculus, m cochlea, n auscultory nerve.)

The vertebrate ear resembles the eye and nose in many important respects, but is different in others, in its development. The auscultory organ in the fully-developed man is like that of the other mammals, and especially the apes, in the main features. As in them, it consists of two chief parts--an apparatus for conducting sound (external and middle ear) and an apparatus for the sensation of sound (internal ear). The external ear opens in the shell at the side of the head (Figure 2.320 a). From this point the external passage (b), about an inch in length, leads into the head. The inner end of it is closed by the tympanum, a vertical, but not quite upright, thin membrane of an oval shape (c). This tympanum separates the external passage from the tympanic cavity (d). This is a small cavity, filled with air, in the temporal bone; it is connected with the mouth by a special tube. This tube is rather longer, but much narrower, than the outer passage, leads inwards obliquely from the anterior wall of the tympanic cavity, and opens in the throat below, behind the nasal openings. It is called the Eustachian tube (e); it serves to equalise the pressure of the air within the tympanic cavity and the outer atmosphere that enters by the external passage. Both the Eustachian tube and the tympanic cavity are lined with a thin mucous coat, which is a direct continuation of the mucous lining of the throat. Inside the tympanic cavity there are three small bones which are known (from their shape) as the hammer, anvil, and stirrup (Figure 2.320, f, g, h). The hammer (f) is the outermost, next to the tympanum. The anvil (g) fits between the other two, above and inside the hammer. The stirrup (h) lies inside the anvil, and touches with its base the outer wall of the internal ear, or auscultory vesicle. All these parts of the external and middle ear belong to the apparatus for conducting sound. Their chief task is to convey the waves of sound through the thick wall of the head to the inner-lying auscultory vesicle. They are not found at all in the fishes. In these the waves of sound are conveyed directly by the wall of the head to the auscultory vesicle.

The internal apparatus for the sensation of sound, which receives the waves of sound from the conducting apparatus, consists in man and all other mammals of a closed auscultory vesicle filled with fluid and an auditory nerve, the ends of which expand over the wall of this vesicle. The vibrations of the sound-waves are conveyed by these media to the nerve-endings. In the labyrinthic water that fills the auscultory vesicle there are small stones at the points of entry of the acoustic nerves, which are composed of groups of microscopic calcareous crystals (otoliths). The auscultory organ of most of the Invertebrates has substantially the same composition. It usually consists of a closed vesicle, filled with fluid, and containing otoliths, with the acoustic nerve expanding on its wall. But, while the auditory vesicle is usually of a simple round or oval shape in the Invertebrates, it has in the Vertebrates a special and curious structure, the labyrinth. This thin-membraned labyrinth is enclosed in a bony capsule of the same shape, the osseous labyrinth (Figure 2.321), and this lies in the middle of the petrous bone of the skull. The labyrinth is divided into two vesicles in all the Gnathostomes. The larger one is called the utriculus, and has three arched appendages, called the "semi-circular canals" (c, d, e). The smaller vesicle is called the sacculus, and is connected with a peculiar appendage, with (in man and the higher mammals) a spiral form something like a snail's shell, and therefore called the cochlea (= snail, b). On the thin wall of this delicate labyrinth the acoustic nerve, which comes from the after-brain, spreads out in most elaborate fashion. It divides into two main branches--a cochlear nerve (for the cochlea) and a vestibular nerve (for the rest of the labyrinth). The former seems to have more to do with the quality, the latter with the quantity, of the acoustic sensations. Through the cochlear nerves we learn the height and timbre, through the vestibular nerves the intensity, of tones.

(FIGURE 2.321. The bony labyrinth of the human ear (left side). a vestibulum, b cochlea, c upper canal, d posterior canal, e outer canal, f oval fenestra, g round fenestra. (From Meyer.)

FIGURE 2.322. Development of the auscultory labyrinth of the chick, in five successive stages (A to E). (Vertical transverse sections of the skull.) fl auscultory pits, lv auscultory vesicles, lr labyrinthic appendage, c rudimentary cochlea, csp posterior canal, cse external canal, jv jugular vein. (From Reissner.))

The first structure of this highly elaborate organ is very simple in the embryo of man and all the other Craniotes; it is a pit-like depression in the skin. At the back part of the head at both sides, near the after brain, a small thickening of the horny plate is formed at the upper end of the second gill-cleft (Figure 2.322 A fl). This sinks into a sort of pit, and severs from the epidermis, just as the lens of the eye does. In this way is formed at each side, directly under the horny plate of the back part of the head, a small vesicle filled with fluid, the primitive auscultory vesicle, or the primary labyrinth. As it separates from its source, the horny plate, and presses inwards and backwards into the skull, it changes from round to pear-shaped (Figures 2.322 B lv and 2.323 o). The outer part of it is lengthened into a thin stem, which at first still opens outwards by a narrow canal. This is the labyrinthic appendage (Figure 2.322 lr). In the lower Vertebrates it develops into a special cavity filled with calcareous crystals, which remains open permanently in some of the primitive fishes, and opens outwards in the upper part of the skull. But in the mammals the labyrinthic appendage degenerates. In these it has only a phylogenetic interest as a rudimentary organ, with no actual physiological significance. The useless relic of it passes through the wall of the petrous bone in the shape of a narrow canal, and is called the vestibular aqueduct.

It is only the inner and lower bulbous part of the separated auscultory vesicle that develops into the highly complex and differentiated structure that is afterwards known as the secondary labyrinth. This vesicle divides at an early stage into an upper and larger and a lower and smaller section. From the one we get the utriculus with the semi-circular canals; from the other the sacculus and the cochlea (Figure 2.320 c). The canals are formed in the shape of simple pouch-like involutions of the utricle (cse and csp). The edges join together in the middle part of each fold, and separate from the utricle, the two ends remaining in open connection with its cavity. All the Gnathostomes have these three canals like man, whereas among the Cyclostomes the lampreys have only two and the hag-fishes only one. The very complex structure of the cochlea, one of the most elaborate and wonderful outcomes of adaptation in the mammal body, develops originally in very simple fashion as a flask-like projection from the sacculus. As Hasse and Retzius have pointed out, we find the successive ontogenetic stages of its growth represented permanently in the series of the higher Vertebrates. The cochlea is wanting even in the Monotremes, and is restricted to the rest of the mammals and man.

The auditory nerve, or eighth cerebral nerve, expands with one branch in the cochlea, and with the other in the remaining parts of the labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal branch of a cerebro-spinal nerve, the motor ventral branch of which acts for the muscles of the face (nervus facialis). It has therefore originated phylogenetically from an ordinary cutaneous nerve, and so is of quite different origin from the optic and olfactory nerves, which both represent direct outgrowths of the brain. In this respect the auscultory organ is essentially different from the organs of sight and smell.