NERVOUS SYSTEM 



441 



are called ncxles of Ranvier, after their discoverer, 

 and are supposed to allow of the percolation of 

 lymph to nourish the axis cylinder. When a nerve 



Fig.2L 



Small part of a nerve-fibre with 

 axis cylinder, surrounded by 

 medullary sheath. The primi- 

 tive sh-ath passes over the 

 constriction in the medullary 

 dMtfe 



Nerve-nbres. stained with 

 nitrate of silver, showing 

 two nodes of Ranvier. 



is divided transversely, and stained appropriately, 

 the axis cylinder appears as a small point sur- 

 rounded by a ring of the whitish medullary sheath 



Fig. 4. 



(tin- 4). This sheath is enclosed by a continuous 

 investment, tlie primitive sJuxtth. This is colour- 

 less, and very delicate, and has a nucleus on its 

 inner side corresponding to each segment of the 

 medullary sheath. In the sympathetic system the 

 medullary sheath is absent ; while the fibres of the 

 brain and spinal cord retain the medullary sheath, 



Fig. 6. Nerve-cells : 



A, from sympathetic ganglion ; B, from cerebrum ; C, from 

 spinal cord ; a.p., axis cylinder process. 



but want the primitive membrane. The sympa- 

 thetic fibres are often called gray or non-medul- 

 lated ; the othera, white or m'edullated. The 

 nerve-fibres in the limbs are about n'jjth of an 



incb in diameter ; in the brain they may be nearly 

 ten times finer. 



The nerve-cells vary greatly in size and in form. 

 Many of them, especially in certain regions of the 

 cortex of the brain, have the shape of an elongated 

 \>\ ramid ( fig. 5, B ), with fine processes coming oft' at 

 various points, others are very irregular in outline, 

 but also with numerous processes, one of which 

 can frequently be traced into continuity with a 

 medullated nerve, and hence is called the axis 

 cylinder process, while the others form a fine net- 

 work before entering another cell or fibre. Such 

 cells are called multipolar ( fig. 5, A and C ), and 

 are seen best in the anterior horns of the gray 

 matter of the spinal cord. Many cells, again, are 

 bipolar i.e. they have only two hbres, one at eacli 

 pole, in connection with them. 



The various end-organs are described under the 



Fig. 6. 



special sections. Fig. 6 shows the manner in 

 which the fibres of a nerve end in a muscle. 



The nerves arising from the brain are arranged 

 in twelve pairs. The first, or olfactory, is the nerve 

 of smell. The second, or optic, is the nerve of si^lit. 

 It arises from the retina, meets with its fellow in the 

 optic chiasma, and is distributed half to each side of 

 the brain, terminating partly in the corpora quadri- 

 gemina (for the reflex movements of the eye), and 

 partly in the optic thalamus, passing thence to the 

 occipital lobe of the cerebrum ( for the sense of 

 sight). The third or ocnlo-motor nerve arises 

 under the corpora qnadrigemina, and passes to all 

 the muscles of the eye except two, which are 

 supplied by the fourth and sixth pairs. The fourth 

 nerve, arising immediately behind the third nerve, 

 supplies the superior oblique muscle of the eye ; 

 while the sixth pair, arising from a nucleus near 

 the middle of the floor of the fourth ventricle, 

 supplies the external rectus muscle of the eye. 

 The fifth pair has a very long origin from a point 

 at the level of the thirn nerve down to the upper 

 part of the spinal cord. It is the motor nerve to 

 the muscles of mastication, and the sensory nerve 

 to the face, front of the head, teeth, tongue, and 

 is the nerve of taste of the anterior part of the 

 tongue. It is this nerve which is concerned in 

 neuralgia of the head and face and teeth. The 

 seventh pair arises from the lower part of the pong 

 Varolii ( see BRAIN ), and is the motor nerve to the 

 facial muscles of expression. Injury to or disease 

 of this nerve causes facial palsy, or Bell's paralysis. 

 The eighth pair, or auditory nerve, supplies the 

 internal ear. It is divided into two parts, one of 

 which supplies the cochlea, and is tin- nerve of 

 hearing proper, while the other supplies the semi- 

 circular canals, and is concerned in the mainten- 

 ance of the equilibrium of the body. The nerve 

 arises from the lateral and posterior part of the 

 pops Varolii and medulla oblongata. The ninth 

 pair, or glosso-pharyngeal nerve, is the special 

 nerve of taste, and supplies the hinder third of 

 the tongue, with the taste bulbs of which it is 

 connected. The tenth pair, or jmeumogastric 

 nerve, has a very wide area of distribution to the 

 lungs, heart, stomach, &c. ; it is partly motor ami 

 partly sensory in function. The eleventh pair, or 

 spinal accessory nerve, is the motor nerve to the 

 larynx, and to certain muscles in the upper part 

 of the neck. These three nerves arise from a 

 groove in the side of the medulla oblongata and 



