TISSUE. 73 



naturally is much greater in the gray than in the white matter. On the other 

 hand, the cholesterin. fat. and cerebrin (the latter ill-defined nitrogenous compounds 

 belonging to the group of glucosides) are probably important constituents of the 

 medullary sheath. Another substance also occurring in the medullary sheath, 

 though not determined separately in the above analysis, is neurokeratin, which 

 forms a fibrous network throughout the sheath, and resembles keratin in its 

 marked resistance to reagents. It probably makes up the greater part of the 

 unidentified organic matter of the white substance in the above analysis, while in 

 the gray substance the unidentified matter is probably largely composed of 

 protagon, a phosphorized compound closely resembling lecithin, but differing from 

 it by its insolubility in ether. 



The nervous structures are divided, as before mentioned, into two great sys- 

 tems viz. the cerebro-spinaL comprising the brain and spinal cord, the nerves 

 connected with these structures, and the ganglia situated on them ; and the sym- 

 pathetic, consisting of a double chain of ganglia and the nerves connected with 

 them. All these structures require separate consideration. 



The brain or encephalon is that part of the cerebro-spinal system which is 

 contained in the cavity of the skull. It is divided into several parts, for a 

 description of which reference must be made to the account of the structure of 

 the brain in a subsequent portion of this work. In these parts the gray matter 

 is found partly on the surface of the brain, forming the cortex of the cerebrum 

 and of the cerebellum. Again, gray matter is found in the interior of the 

 brain, collected into large and distinct masses or ganglionic bodies, such as the 

 corpus striatum. optic thalamus, corpora quadrigemina, the olivary bodies, and 

 the corpora dentata of the cerebellum. Finally, gray matter is found intermin- 

 gled intimately with the white, but without definite arrangement, as in tlie 

 gray matter in the pons Yarolii and the floor of the fourth ventricle. 



The white matter of the brain is divisible into three distinct classes of fibres. 

 These are. in the first place, projection fibres, such as the fibres which connect 

 the brain with the spinal cord ; that is to say, those which are usually traced 

 upward from the columns of the spinal cord, through the medulla oblongata into 

 the encephalon. chiefly by means of the anterior pyramids, passing through the 

 pons Yarolii and crura cerebri to the internal capsules of the corpora striata, 

 and thence to the cerebral cortex, and by means of the restiform bodies into the 

 cerebellum. The second class of white fibres in the brain are commissural, con- 

 necting opposite sides of the brain, as. for instance, the fibres of the corpus callo- 

 sum and the anterior commissure of the thalamencephalon. And the third class 

 are the association fibres which connect different regions of the same side of any 

 of the portions of the brain. The fibres of this last class are more especially 

 developed in the cerebral hemispheres, where they connect different areas of the 

 cortex, as. for example, the cortical centre for sight in the occipital lobe with the 

 motor centre for speech in the frontal lobe. 



The manner in which the gray and white matter are intermingled in the brain 

 and spinal cord is very intricate, and can only be fully understood by a careful 

 study of the details of its descriptive anatomy in the sequel. The further consid- 

 eration of this subject will therefore be deferred until after the description of the 

 various divisions of which the cerebro-spinal system is made up. 



The nerves are round or flattened cords, formed of the nerve-fibres already 

 described. They are connected at one end with the cerebro-spinal centre or with 

 the ganglia, and are distributed at the other end to the various textures of the 

 body : they are subdivided into two great classes the cerebro-spinal, which pro- 

 ceed from the cerebro-spinal axis, and the sympathetic or ganglionic nerves, which 

 proceed from the ganglia of the sympathetic. The cerebro-spinal nerves consist 

 of numerous nerve-fibres collected together and enclosed in a membranous sheath 

 (Fig. 51). A small bundle of primitive fibres, enclosed in a tubular sheath, is 

 called a funicuhis : if the nerve is of small size, it may consist only of a single 

 funiculus ; but if large, the funiculi are collected together into larger bundles or 



