721. The uses of the hair vary in different regions of the body. Upon the head, it aids in shielding the brain from injury by blows, and it likewise serves to protect this part of the system from heat and cold, thus maintaining equal temperature of the cerebral organ. About the flections of the joints, as in the axilla, (armpit,) they prevent irritation of the skin from friction; in the pa.s.sages to the ears and nostrils, they present an obstacle to the ingress of insects and foreign bodies; while in the eyebrows and eyelids, they serve to protect the organ of vision.

[Ill.u.s.tration: Fig. 119. A section of the end of the finger and nail. 4, Section of the last bone of the finger. 5, Fat, forming the cushion at the end of the finger. 2, The nail. 1, 1, The cuticle continued under and around the root of the nail, at 3, 3, 3.]

722. The NAILS are hard, elastic, flexible, semi-transparent scales, and present the appearance of a layer of horn. The nail is divided into the _root_, the _body_, and the _free portion_. The root is that part which is covered on both surfaces; the body is that portion which has one surface free; the free portion projects beyond the end of the finger.

723. The nail is formed of several lamin, or plates, that are fitted the one to the other; the deepest is that which is last formed. The nails, as well as the hoofs of animals and the cuticle, are products of secretion. They receive no blood-vessels or nerves. If the cuticle be removed in severe scalds they will separate with it, as the hoofs of animals are removed by the agency of hot water. The nails increase in length and thickness, by the deposition of alb.u.men upon their under surface, and at their roots, in a manner similar to the growth of the cuticle, of which they const.i.tute a part.

How can it be spread along the hairs? Why should soap not be used in washing the hair? 721. Of what use is the hair upon the head? About the flexions of the joints? In the nasal and ear pa.s.sages? Upon the eyebrows and eyelids? 722. Describe the nails. 723. How are they formed?

_Observations._ 1st. The nail upon its under surface is fashioned into thin vertical plates, which are received between the folds of the sensitive skin. In this manner, the two kinds of lamin reciprocally embrace each other, and the firmness of connection of the nail is maintained. If we look on the surface of the nail, we see an indication of this structure in the alternate red and white lines which are there observed. The former of these correspond with the sensitive lamin; the latter with the h.o.r.n.y plates. The ribbed appearance of the nail is due to the same circ.u.mstance. These sensitive lamin are provided with an unusual number of capillary vessels for the formation of the nail, and hence they give a red tint to the portion under which they lie.

2d. Near the root of the nail there is a part that is not laminated, and it is less abundantly supplied with blood-vessels. This portion consequently looks pale compared with the laminated portion, and from its half-moon shape is technically termed _lunula_. Beyond the lunula, the root of the nail is imbedded in the fold of the sensitive skin, and has the same relation to that structure that any single one of the thin h.o.r.n.y plates of its under surface has to its corresponding pair of sensitive lamin.

724. The nails, from their position, are continually receiving knocks, which produce a momentary disturbance of their cell formation, followed by a white spot. The care of the nails should be strictly limited to the knife or scissors, to their free border, and an ivory presser, to prevent adhesion of the free margin of the scarf-skin to the surface of the nail. This edge of the cuticle should never be pared, the surface of the nail never sc.r.a.ped, nor the nails cleaned with any instrument whatever, except the nail-brush, aided by water and soap. An observance of these suggestions, will prevent irregularities and disorders of the nails.

Give observation 1st. Observation 2d. 724. How should the nails be treated to prevent irregularities and disease?

_Observations._ 1st. When we wear a shoe that is too short for the foot, the edge of the nail is brought against the leather. This interrupts the forward growth of the nail, and it spreads out on the sides, and becomes unusually thick. It then presses upon the soft parts, and is said to "grow into the flesh." The prevention is, to wear shoes of ample size.

2d. Instances are by no means unfrequent in which the power of production of the nail at the root becomes entirely destroyed, and it then grows in thickness only. When this affection occurs, it is often remarkable what a ma.s.s the nail presents. Instances are on record, where the nail is regularly shed; and, whenever the old nail falls off, a new one is found beneath it, perfectly formed. Sometimes the growth in length is not entirely checked, although growth in thickness is induced; the nail then presents a peculiar appearance.

What causes the edge of the nail "to grow into the flesh" of the toe?

How prevented?

CHAPTER x.x.xVI.

THE NERVOUS SYSTEM.

725. In the preceding chapters, we have seen how various and complex are many of the motions necessary to maintain the life of an animal whose organization is superior to all others. We have noted the wonderful mechanism of the muscular system, in producing the varied movements of the body, the different processes by which the food is converted into chyle and mixed with the blood, and the circulation of this fluid to every organ and tissue of the system, that each may select from it the very principles which it requires for its growth.

726. Lymphatic absorption commences as soon as nutrition is completed, and conveys the useless, worn-out particles of different tissues back into the circulating fluid; while the respiratory organs and secretory glands perform the work of preparing the waste products to be eliminated from the body. Each of these processes effects a single object, and is performed in a regular manner.

727. "They must succeed each other in proper order in propelling every particle to its proper destination, or life would be sacrificed almost at the moment of its commencement. There is, therefore, a mutual dependence of all portions of the machinery of organic life upon each other, and a necessity for some medium of communication from one organ to another, by which they may convey mutual information of their several conditions, if we may be permitted to employ a figurative expression. Were there no such medium, how would the stomach notify the heart that additional exertion on its part is required, because the stomach is busy in digesting food?

725. What has been noted in the preceding chapters? 726. Show the manner in which the several processes are performed. 727. How must they succeed each other?

728. "When we are exerting the muscular system for a long time in some laborious employment, how else are our members to inform the stomach that they are too much occupied with their duties to spare the blood necessary in digestion; that it is requisite that the appet.i.te should decline; and that digestion should cease for the time, even if the stomach should be oppressed with its contents? When we are thinking, how else are the blood-vessels to be told that an unusual supply of their contents is wanting in the head? or when the whole frame is weary with exertion, how, without some regular line of intelligence between all the organs, is the brain to be instructed that circ.u.mstances require that it should go to sleep? To supply the necessary medium of communication, Providence has furnished all the animals that possess distinct organs, with a peculiar apparatus called the _Nervous System_."

ANATOMY OF THE NERVOUS SYSTEM.

729. The NERVOUS SYSTEM consists of the _Cere-bro-spinal Centre_, and of numerous rounded and flattened white cords, called _nerves_, which are connected at one extremity with the cerebro-spinal centre, and at the other, distributed to all the textures of the body. The sympathetic nerve is an exception to this description; for, instead of one, it has many small centres, which are called _gangli-a_, and which communicate very freely with the cerebro-spinal centre, and with its nerves.

728. What is the medium of communication from one organ to another?

729-754. _Give the anatomy of the brain and cranial nerves._ 729. Of what does the nervous system consist? What const.i.tutes an exception to this?

730. The CEREBRO-SPINAL CENTRE consists of two portions: The _brain_, and the _spinal cord_. For convenience of description, the nervous system may be divided into the _Brain_, _Cranial Nerves_, _Spinal Cord_, _Spinal Nerves_, and the _Sympathetic Nerve_.

731. The term BRAIN designates those parts of the nervous system, exclusive of the nerves themselves, which are contained within the cranium, or skull-bones; they are the _Cere-brum_, _Cer-e-bellum_, and _Me-dulla Ob-lon-gata_. These are invested and protected by the membranes of the brain, which are called the _Dura Mater_, _A-rachnoid_, and _Pia Mater_.

[Ill.u.s.tration: Fig. 120. 1, 1, The scalp turned down. 2, 2, 2, The cut edge of the bones of the skull. 3, The external strong membrane of the brain (dura mater,) suspended by a hook. 4, The left hemisphere of the brain, showing its convolutions.]

730. Of what does the cerebro-spinal centre consist? How is the nervous system divided? 731. What does the term brain designate? Name them. How are they protected? Describe fig. 120.

732. The CEREBRUM IS divided into two hemispheres, by a cleft, or fissure. Into this cleft dips a portion of the dura mater, called the _falx cere-bri_, from its resembling a sickle. The apparent design of this membrane is to relieve the one side from the pressure of the other, when the head is reclining to either side. Upon the superior surface of the cerebrum are seen undulating windings, called _con-vo-lutions_. Upon its inferior, or lower surface, each hemisphere admits of a division into three lobes--the anterior, middle, and posterior. (Fig. 122, 123)

[Ill.u.s.tration: Fig. 121 A section of the skull-bones and cerebrum. 1, 1, The skull. 2, 2, the dura mater 3, 3, The cineritious portion of the cerebrum. 4, 4, The medullary portion. The dark points indicate the position of divided blood-vessels. 5, 5, The lateral ventricles.]

732. How is the cerebrum divided? What is the use of the falx cerebri?

What is seen upon the superior surface of the brain? Its inferior?

733. When the upper part of the hemispheres is removed horizontally with a sharp knife, a centre of white substance is brought to view.

This is surrounded by a border of gray, which follows the depressions of the convolutions, and presents a zigzag outline. The divided surface will be seen studded with numerous small red points, which are produced by the escape of blood from the division of the minute arteries and veins. The gray border is called the cortical, or _cineritious_ portion, while the white central portion is called the _medullary_. The two hemispheres are connected by a dense layer of transverse fibres, called _corpus cal-losum_.

734. In the interior of the brain there are several cavities, two of which are of considerable size, and are called the lateral ventricles.