Part 3 (1/2)

THE CENTRAL SYSTEM--In the brain we easily distinguish three major divisions--the _cerebruata_

The ed upper part of the cord where it connects with the brain It is about an inch and a quarter long, and is composed of both medullated and unray” matter In the medulla, the unmedullated neurones which co to the outside, and thethe positions they occupy in the cerebru sides, so that those which pass up the right side of the cord finally connect with the left side of the brain, and vice versa

THE CEREBELLUM--Lying just back of the medulla and at the rear part of the base of the cerebrum is the cerebellue as the fist, and coray matter Fibers froe and pass on into the cerebrum, while its two halves also are connected with each other by means of cross fibers

[Illustration: FIG 8--View of the under side of the brain B, basis of the crura; P, pons; Mo, ata; Ce, cerebellum; Sc, spinal cord]

THE CEREBRUM--The cerebrum occupies all the upper part of the skull from the front to the rear It is divided syht and the left These hee of fibers called the _corpus callosued with convolutions, an arrangereater surface for the distribution of the gray cellular ularities of surface, each hemisphere is marked also by two deep clefts or _fissures_--the fissure of Rolando, extending from the middle upper part of the he a little in front of the ear and stopping on a level with the upper part of it; and the fissure of Sylvius, beginning at the base of the brain so upward and backward at an acute angle with the base of the hera cerebelluata (MO) F' F” F”' are placed on the first, second, and third frontal convolutions, respectively; AF, on the ascending frontal; AP, on the ascending parietal; M, on the ular T' T” T”' are placed on the first, second, and third temporal convolutions R-R marks the fissure of Rolando; S-S, the fissure of Sylvius; PO, the parieto-occipital fissure]

The surface of each heht of as mapped out into four lobes: The frontal lobe, which includes the front part of the hemisphere and extends back to the fissure of Rolando and down to the fissure of Sylvius; the parietal lobe, which lies back of the fissure of Rolando and above that of Sylvius and extends back to the occipital lobe; the occipital lobe, which includes the extreme rear portion of the hemisphere; and the temporal lobe, which lies below the fissure of Sylvius and extends back to the occipital lobe

THE CORTEX--The gray matter of the hemispheres, unlike that of the cord, lies on the surface This gray exterior portion of the cerebrum is called the _cortex_, and varies frohth of an inch in thickness The cortex is the seat of all consciousness and of the control of voluntary movement

[Illustration: FIG 10--Different aspects of sections of the spinal cord and of the roots of the spinal nerves froion: 1, different views of anterior median fissure; 2, posterior fissure; 3, anterior lateral depression for anterior roots; 4, posterior lateral depression for posterior roots; 5 and 6, anterior and posterior roots, respectively; 7, complete spinal nerve, formed by the union of the anterior and posterior roots]

THE SPINAL CORD--The spinal cord proceeds froh a canal provided for it in the vertebrae of the spinal coluray matter within A deep fissure on the anterior side and another on the posterior cleave the cord nearly in twain, reseray matter on the interior is in the form of two crescents connected by a narrow bar

The _peripheral_ nervous systeans, branching off from the cord, and twelve pairs that have their roots in the brain Branches of these forty-three pairs of nerves reach to every part of the periphery of the body and to all the internal organs

[Illustration: FIG 11--The projection fibers of the brain I-IX, the first nine pairs of cranial nerves]

It will help in understanding the peripheral system to remember that a _nerve_ consists of a bundle of neurone fibers each wrapped in its medullary sheath and sheath of Schwann Around this bundle of neurones, that is around the nerve, is still another wrapping, silvery-white, called the neurile to make up a nerve varies from about 5,000 to 100,000 Nerves can easily be identified in a piece of lean beef, or even at the edge of a serious gash in one's own flesh!+

Bundles of sensory fibers constituting a sensory nerve root enter the spinal cord on the posterior side through holes in the vertebrae Similar bundles of e froence froether in the same sheath and proceed in this way to the periphery of the body, where the sensory nerve usually ends in a specialized _end-organ_ fitted to respond to some certain stimulus from the outside world The an which it governs Both sensory and motor nerves connect with fibers of like kind in the cord and these in turn with the cortex, thus giving every part of the periphery direct connection with the cortex

[Illustration: FIG 12--Sche cortical centers with each other--After JAMES and STARR]

The _end-organs_ of the sensory nerves are nerve ue, relatively simple; and others, as the eye or ear, very complex They are all alike in one particular; namely, that each is fitted for its own particular work and can do no other

Thus the eye is the end-organ of sight, and is a wonderfully co ht The ear has for its essential part the specialized endings of the auditory nerve, and is fitted to respond to the waves carried to it in the air, giving the sensation of sound The end-organs of touch, found in greatest perfection in the finger tips, are of several kinds, all very complicated in structure And so on with each of the senses Each particular sense has soan specially adapted to respond to the kind of stimulus upon which its sensation depends, and each is insensible to the stimuli of the others, much as the receiver of a telephone will respond to the tones of our voice, but not to the touch of our fingers as will the telegraph instrument, and _vice versa_ Thus the eye is not affected by sounds, nor touch by light Yet by ether we are able to come in contact with the material world in a variety of ways

5 LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM

DIVISION OF LABOR--Division of labor is the law in the organic world as in the industrial Animals of the lowest type, such as the aestion, assi all of these functions But in the higher foran not only has its own specific work, but even within the saned Thus we have seen that the two parts of the neurone probably perfory and the fibers transe, then, that there is also a division of labor in the cellular matter itself in the nervous systelia which are distributed at intervals along the nerves are probably for the purpose of reenforcing the nerve current, raph office reenforce the current from the central office The cellular matter in the spinal cord and lower parts of the brain has a very ies fro the simpler reflex acts and movements which we learn to execute without our consciousness being called upon, thus leaving the her ways The cellular hest functions of all, for through its activity we have consciousness

[Illustration: FIG 13--Side view of left he the principal localized areas]

The gray matter of the cerebellum, the medulla, and the cord may receive impressions from the senses and respond to them with movements, but their response is in all cases wholly automatic and unconscious A person whose hemispheres had been injured in such a way as to interfere with the activity of the cortex ht still continue to perform most if not all of the habitual movements of his life, but they would be her consciousness

It is through the activity of this thin covering of cellular matter of the cerebrum, the _cortex_, that our minds operate; here are received stimuli from the different senses, and here sensations are experienced

Here all our in

And here all our thinking, feeling, and willing are done

DIVISION OF LABOR IN THE CORTEX--Nor does the division of labor in the nervous systenment of work The cortex itself probably works essentially as a unit, yet it is through a shi+fting of tensions fro us a sensation, now directing aan emotion Localization of function is the rule here also