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  Section: Anatomy of Vertebrate Animals » The Muscles and the Viscera
 
 
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The Sensory Organs

 
     
 

The organs of the three higher senses - Smell, Sight, and Hearing - are situated, as has been already described, in pairs, upon each side of the skull, in all vertebrate animals except the lowest fishes; and, in their earliest condition, they are alike involutions of the integument.

The Olfactory Apparatus acquires no higher complication than this, being either a single sac (Amphioxus (?) Marsipobranchii), or, more commonly, two, the surfaces of which are increased by plaiting, or by the development of turbinal cartilages, or bones, from the lateral portions of the ethmoid. Upon these, nervous filaments arising from the olfactory lobe of the brain are distributed. The cavities of the olfactory sacs may be placed in communication with that of the mouth by the nasal passages; or, as in the great majority of fishes, they may have only an external aperture, or apertures.

In Reptiles, Birds, and Mammals, a peculiar nasal gland is frequently connected with, and pours its secretion into, each olfactory chamber.

The foramina incisiva, left between the premaxillaries and the palatine plates of the maxillaries in Mammalia, are sometimes closed by the mucous membranes of the nasal and oral cavities, and sometimes not. In the latter case they are the canals of Stenson, aud place these two cavities in communication. Glandular diverticula of the mucous membrane, supplied with nervous filaments from both the olfactory and the fifth pair, may open into these canals. They are called, after their discoverer, the "organs of Jacobson."

The Eye is formed by the coalescence of two sets of structures, one furnished by involution of the integument, the other by an outgrowth of the brain.

The opening of the integumentary depression which is primarily formed on each side of the head in the ocular region becomes closed, and a shut sac is the result. The outer wall of this sac becomes the transparent cornea of the eye; the epidermis of its floor thickens, and is metamorphosed into the crystalline lens; the cavity fills with the aqueous humor. A vascular and muscular ingrowth taking place round the circumference of the sac, and, dividing its cavity into two segments, gives rise to the iris. The integument around the cornea, growing out into a fold above and below, results in the formation of the eyelids, and the segregation of the integument which they enclose, as the soft and vascular conjunctiva. The pouch of the conjunctiva very generally communicates, by the lachrymal duct, with the cavity of the nose. It may be raised, on its inner side, into a broad fold, the nictitating membrane, moved by a proper muscle or muscles. Special glands - the lachrymal externally, and the Harderian on the inner side of the eyeball - may be developed in connection with, and pour their secretion on to, the conjunctival mucous membrane.

The posterior chamber of the eye has a totally distinct origin. Very early, that part of the anterior cerebral vesicle which eventually becomes the vesicle of the third ventricle, throws out a diverticulum, broad at its outer, and narrow at its inner end, which applies itself to the base of the integumentary sac. The posterior, or outer, wall of the diverticulum then becomes, as it were, thrust in, and forced toward the opposite wall, by an ingrowth of the adjacent connective tissue; so that the primitive cavity of the diverticulum, which of course, communicates freely with that of the anterior cerebral vesicle, is obliterated. The broad end of the diverticulum acquiring a spheroidal shape, while its pedicle narrows and elongates, the latter becomes the optic nerve, while the former, surrounding itself with a strong fibrous sclerotic coat, remains as the posterior chamber of the eye. The double envelope, resulting from the folding of the wall of the cerebral optic vesicle upon itself, gives rise to the retina and the choroid coat: the plug, or ingrowth of connective tissue, gelatinizes and passes into the vitreous humor, the cleft by which it entered becoming obliterated.

Even in the higher Vertebrata the optic nerve is, at first, connected exclusively with the vesicle of the third ventricle, and makes no decussation with its fellow. But by degrees the roots of origin of each nerve extend over to the opposite side of the brain, and round the thalamus, to the mesencephalon on that side, and the trunks of the two nerves become intermixed below the third ventricle, in a close and complicated manner, to form a chiasma.

In Amphioxus and Myxine, the eyes are very imperfeclly developed, appearing to consist of little more than a rudimentary lens imbedded in the pigment, which encloses the termination of the optic nerve; and, in Myxine, this rudimentary eye is hidden by muscles and integument. It appears doubtful whether in these fishes, and in the Lampreys, the eye is developed in the same way as in other Vertebrata.

In all other Vertebrata, the eyes have the typical structure, though sometimes, as in the Blind - fish (Amblyopsis) and the Mole, they have no functional importance. In the Ichthyopsida and Sauropsida, but not in Mammalia, the sclerotic is often partially ossified, the ossification usually forming a ring around its anterior moiety. It becomes enormously thickened in the Cetucea.

Except in Amphioxus and the Myxinoid fishes, the eye ball is moved by six muscles; of these, four, proceeding from the interior of the orbit to the periphery of the eyeball, and surrounding the optic nerve, are termed superior, inferior, internal, and external recti. The other two are connected with the upper and the lower margins of the orbit respectively, and pass thence to the outer side of the bulb. These are the superior and the inferior obliqui. In many Reptiles and Mammals a continuous funnel - shaped sheet of muscle, the musculus choanoides, lies within the four recti, and is attached to the circumference of the posterior moiety of the ball of the eye. It would appear, from the distribution of the nerves, which has already been described, that the musculus choanoides, the external rectus, and the nictitating muscle, constitute a group of eye-muscles morphologically distinct from the other three recti, the obliqui, and the levator palpebrae superioris. In many Reptiles, and in the higher Vertebrata, the eyelids are closed by circular muscular fibres, constituting an orbicularis palpebrarum, and are separated by straight fibres proceeding from the back of the orbit, usually to the upper eyelid only, as the levator palpebrae superioris; but sometimes to both lids, when the lower muscle is a depressor palpebrae inferioris.

The Harderian and lachrymal glands are not found in fishes; but the former is met with in the Batrachia, and both are of common occurrence in the Sauropsida and Mammalia.

In Lacertilia, Crocodilia, Aves, and many Fishes, a peculiar vascular membrane, covered with pigment, like the choroid, projects from near the entrance of the optic nerve, on the outer side of the globe of the eye, into the vitreous humor, and usually becomes connected with the capsule of the lens. This is the pecten, or marsupium.

 
     
 
 
     



     
 
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