Part 47 (2/2)

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.

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