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 appetite 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 circumstances 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 _Cer´e-bro-spi´nal Cen´tre_, 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 _gan´gli-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 constitutes 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 _Cer´e-brum_, _Cer-e-bel´lum_, and _Me-dul´la Ob-lon-ga´ta_. These are invested and protected by the membranes of the brain, which are called the _Du´ra Ma´ter_, _A-rach´noid_, and _Pi´a Ma´ter_. [Illustration: 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. -=-=-=-=-=-=-=-=-=-=-=-=