Page:The New International Encyclopædia 1st ed. v. 14.djvu/441

NERVOUS SYSTEM. 301 NERVOUS SYSTEM. i entire liraiii. Xlie maiiimaliun brain is char- atterizcd by tbc presence of large cummissures, the eurjius ealiosum and fornix, which connect the lienuspheres. Xlie jjons Varolii is another large commissure and connects the two lateral lobes of the cerebellum, and a number of other bands serve as connectives. In mammals the mid-brain is relatively smaller than in other vertel)rates. The brain of Tertiary manunals was relatively small. The reduction was especially evident in the hemispheres. Jn many points of structure their brain, too, was reptilian in char- acter, although the possessors were related to groups as high as ungulates. Three main and well-defined types of nervous system may be determined in the animal king- dom. There is first the radial type, such as has been described for the starfish. This consists of a ring of nervous tissue in the disk which surroiuuls the oesophagus and sends out a branch to each arm. This is the echinoderm type. An- other type is the symmetrical, bilateral type of annelids and arthropods, to which tlie nervous system of moUusks may also be relegated. This type consists of a brain ganglion above the esoph- agus, with commissures that pass around and below the digestive tract, where they unite ven- trally with a chain of ganglia extending posteri- orly. The pedal ganglion of mollusks repre- sents the subcesophageal ganglion. The rest of the ventral chain is absent in unsegmented mol- lusks. The third type of nervous system is the bilateral type of vertebrates. Here the double nerve cord, segmental in origin, as is shown by the repetition of paired spinal nerves, lies on the dorsal side of the alimentary tract. Except in Amphioxus the vertebrate nerve cord widens anteriorly into a large and complex ganglion mass, the brain. The sympathetic system, which appears first in leeches and arthropods, is an almost independent nervous system, for while it is in direct union with the other nervous system, it is not at all under its control. Sense Orciaxs. As soon as sense organs ap- pear the nervous elements of animals consist of three parts: (1) the so-called terminal, end, or sense organ; (2) the ganglia; and (.3) the nerves. End organs appear first clearly defined in the Coelenterata, although the pigment spots of Euglena are in a particularly sensitive region in that infusorian. The sense organs of the Coe- lenterates are most developed in the medusa> and consist of the tentaculocysts with calcareous lith- ites. pigiuent spots or ocelli, and sensory pits known as olfactory pits. Pigment cells or 'eye spots' are found in many of the flatworms, some of the polyclads possessing several hundred. These eyes, in many forms, are grouped in pairs at the anterior end of the body or are scattered along the anterior margin, or even along the entire margin of the body. In many forms the eyes are simple i)igment spots. In others a re- fractive body occurs. In triclads and polyclads the eye consists of a pigmented apparatus with retinal or light-perceiving cells within a cup- shaped organ. These cells connect with the optic nerve. Auditory organs also occur in the flatworms. each consisting of a fluid-filled sphere lying close to the brain in which is an otolith. The skin is everywhere very sensitive, due to tactile hairs, especially on the tentacles. The proboscis of certain forms contains an invagina- tion which is capable of extrusion, and is be- lieved to be a touching organ. The suckers of Irematodes, too, are especially sensitive. There are ciliated pits in the epithelium of triclads and other forms to which an olfactory function luis been ascribed. Sense organs occur over the whole integument of mollusks. These cells, when in exposed posi- tions, may have a tactile function. The pallial sensory organs and the ospharadium, which occur near the gills, are probably olfactory organs to test the condition of the water. The optic tentacles of pulmonates and the dorsal ten- tacles of opisthobranchs are stated by some au- thorities to be olfactorj- organs. Ciliated ridges in the mantle furrows of certain chitons are supposed to have an olfactory function. Auditory organs are absent in the Amphineura alone among mollusks. The auditory organ consists of a closed, fluid-filled vesicle, or 'otocyst,' in which a varying uiunber of otoliths are inclosed. Sensory cells occur on the wall of the otoc^-st, which bear sensory hairs. The otocysts of cephalopods have been shown to have a statjcal function also. The simplest visual organs of mollusks are optic pits. The retina lies at the base of the pit, and along this base the optic nerve spreads out. The retina consists of cells with and without pigment. External to the retina is a gelatinous cuticle or even a lens. By the approach and fusion of the edges of the pit the vesicular eye is formed. That part of the epithelium which is free from pigment consti- tutes the cornea. This type of eye occurs on the tentacles of gastropods. By a further develop- ment the simple cup-like eye is differentiated into the complex, highly developed organ of cephalo- pods. The mantle eyes of the scallop resemble vertebrate eyes in that the visual rods are turned awaj' from the light, that is to say, they are directed toward the body. The sense organs of echinoderms comprise the auditory organs and eyes. The auditory organs are of two types. The first type is represented in holothurians, and consists of an otocyst which is lined with ciliated epithelium, container and otolith, and filled with fiuid. In echinoids is found the second type of auditory or orienting organs, which are believed to be transformed spines. Organs of this type are known as 'sphirridia,' and are developed only on the oral side ; they are spherical. stalked bodies and may be inclosed in a pit of the test. The organs of special sense in the annelids are the eyes, ten- tacles, cirri, and otocysts. The structure of the eye, when that organ is present, is very simple. The eyes are confined to the prostomium, but there are eye-like organs on many or all the body segments or on the branchial filaments of many species. Eyes are lacking in nearly all Oligochipta and some of the Polycha-ta. The eye is usually a capsule with a layer of elongated cells on the side toward the brain. A thickening of the cuticle may form a lens. Otocysts with otoliths are not common. Tactile cells occur generally all over the bod.v and especially on the prostomium. The organs of special sense in arthropods are eyes, olfactory seL-e, auditory organs, and organs of touch. The eyes are of two sorts, median eyes and a pair of faceted, compound eyes. The median or 'nauplius' eyes sometimes occur in the larvne only, and arc composed of a cuticular lens and a group of specially modified epidermal cells.