CHAMBERS'S INFORMATION FOR THE PEOPLE. 



cerebellum, and ganglia, we also find tubes running 

 from their anterior to their posterior ends, while 

 other tubes run transversely, and unite correspond- 

 ing parts on opposite sides of the brain. Thus 

 there is evidently community of function. 



The functions of these different parts may be 

 briefly stated to be as follows : i, the cerebrum is the 

 seat of Sensation, Volition, Emotion, and of those 

 intellectual powers which constitute MIND ; 2, the 

 cerebellum is probably the regulator of muscular 

 movements ; 3, the corpora striata is a great centre 

 of voluntary movement, not of volition, but of the 

 nervous mechanism by which, when we will with 

 the cerebrum, the influences are sent along the 

 spinal cord to the various muscles ; 4, the optic 

 thalami perform the same function with regard to 

 sensation ; 5, the corpora quadrigemina receive 

 visual impressions by the optic nerves from the 

 retina of the eyes, and transmit these to the cere- 

 brum, where there is then the consciousness of 

 sight ; and 6, the medulla oblongata, and an 

 adjoining part called the pans Varolii, are the 

 seat of the nervous influences which regulate 

 swallowing, breathing, and other important invol- 

 untary movements. The parts of the brain last 

 mentioned (6) are absolutely essential to life. The 

 other parts may be cut or mutilated without in- 

 stant death, but this surely follows in a few 

 moments injury to the medulla. 



Nerves have different functions. When an in- 

 fluence travels along a nerve to a muscle, it excites 

 the muscle to contract, and the former is then 

 called a motor nerve ; when it travels to the brain 

 and causes a sensation, we call such a nerve sen- 

 sory. Most nerves contain both sensory and motor 

 tubes. Some are purely sensory, as certain parts 

 of the fifth cranial nerve ; others purely motor, as 

 the facial, or seventh cranial nerve ; while all the 

 spinal nerves are senso-motory. Certain nerves 

 respond only to particular stimuli. For example, 

 the optic nerve is affected only by vibrations of 

 rays of light. Such are called special sensory 

 nerves, and include those of sight, hearing, taste, 

 smell. The nerves of touch are those of common 

 sensibility distributed to the skin. 



Twelve pair of nerves are given off from the 

 brain, and thirty-one from the spinal cord. 



The spinal cord or marrow is a cylindrical 

 column of soft nervous tissue, extending from the 

 base of the skull, where it is continuous with the 

 medulla oblongata, to the region of the loins, 

 where it tapers off to a thread in the lowest part 

 of the vertebral canal. Its average length is 

 eighteen inches. It is not only divided by two 

 fissures in the middle, but each half is again 

 divided longitudinally into three equal parts by 

 two parallel series of nervous filaments, which are 

 the anterior and posterior roots of the spinal 

 nerves. The posterior root presents a swelling or 

 ganglion, immediately beyond which the two 

 coalesce into the trunk of a nerve which, after 

 emerging through a hole called the intervertebral 

 foramen, is distributed into branches to the parts 

 it is destined to supply with nervous filaments ; 

 as, for example, the muscles of the trunk and 

 limbs and the surface of the body. These roots 

 have separate functions, the anterior being com- 

 posed of motor, while the posterior contain 

 sensory tubes. Hence if the anterior root (in 

 a vivisection operation) is divided, or if the 

 column of the cord from which it springs is dis- 



124 



eased, loss of motion in the part which it sup- 

 plies is the result ; while if the posterior root were 

 similarly acted on, there would be loss of sen- 

 sation. The anterior columns of the medulla 

 decussate (that is, send nerve-tubes across to the 

 adjoining column) ; while many of the tubes of 

 the posterior columns decussate all the way up 

 the back of the cord. Consequently, injury to 

 the right anterior column causes loss of motion 

 on the same side, while injury to the right pos- 

 terior column paralyses sensation as regards the 

 opposite side of the body. The decussation of the 

 anterior columns in the medulla also explains how 

 a clot of blood in the right hemisphere of the 

 brain, as in apoplexy, causes loss of motor-power 

 on the left side of the body. 



THE SPECIAL SENSES. 



There' are five senses namely, touch, taste, 

 smell, sight, and hearing: 



Touch. The sense of touch, including that of 

 different degrees of heat, is possessed by the 

 skin, by the walls of the mouth and nostrils, and 

 by the tongue, but it is most highly developed 

 on the tips of the fingers. The essential organs 

 of this sense are the true skin, containing capil- 

 laries, and the terminations of sensory nerves. On 

 examining the surface of the true skin by a magni- 

 fying-glass, we can see a regular arrangement of 

 papillae, or cone-like projections about rrsth 

 of an inch in length. In many of these papillae 

 there are found small round or oval bodies 

 made of hard fibrous tissue, and having a nerve- 

 tube coiled round them, and sometimes pene- 

 trating into their interior. These are called touch- 

 bodies. They serve as resisting structures against 

 which the nerve may be pressed, and thus the sense 

 of touch may be intensified. When one of those 

 nerves is pressed by the contact of any foreign 

 body, an influence is produced which travels to 

 the brain, where we become conscious of the im- 

 pression. This consciousness, however, we refer not 

 to the brain, but to the part affected ; a subjective 

 power, which is probably the result of experience. 



Taste. The organs of taste are in the mucous 

 membrane of the tongue, especially at its back 

 part. The nerves of taste are the lingual branch 

 of the fifth cranial nerve and the glosso-pharyn- 

 geal, the former supplying the anterior two-thirds, 

 and the latter the posterior one-third of the tongue. 

 The mucous membrane of the tongue presents 

 papillae of various forms, called _/f/z/0;v#, or thread- 

 like ; fungiform, or mushroom-like ; and circum- 

 vallate. The circumvallate are about thirteen or 

 fifteen papillae set in the form of a V with its 

 point backwards, and each resembles a fungiform 

 papillae surrounded by a wall. These last-named 

 structures are regarded as the essential organs of 

 taste. They derive their nerves chiefly from the 

 glosso-pharyngeal nerve. The contact of a sapid 

 body with the surface of the tongue is not sufficient 

 to evoke the sense of taste. The substance must 

 be dissolved, and to effect its solution nature pro- 

 vides a special fluid the saliva. In febrile dis- 

 eases, in which the tongue is dry and coated, the 

 sense of taste is either dormant or perverted. 

 Taste is more acute in some persons than in 

 others. It is sometimes blended in a remarkable 

 way with smell, giving rise to the peculiar sensa- 

 tion we call flavour. The sensations produced by 





