224 



INNER VATION. 



[CHAP. vm. 



muscles which derive their nerves from this source, and that destruc- 

 tive disease of that organ may occasion paralysis of those muscles. 



Of the Nervous Centres. The nervous centres exhibit to us the 

 union of the vesicular and the nervous fibrous matter. Indeed 

 the association of these two forms of nervous sutatance in a mass of 

 variable shape or size is the main anatomical condition for the 

 formation of a nervous centre. The former is never met with in 

 nerves properly so-called, and when a true nerve has a grayish ap- 

 pearance, we find that it is owing to a paucity of the tubular, and 

 an excess of the gelatinous fibres. 



All nervous centres are provided with a proper covering which 

 serves to isolate them from adjacent textures and to protect them, 

 as well as to support their nutrient blood-vessels. In the ganglia 

 this covering is continuous and identical with the neurilemma of the 

 nerves which are connected with them, and it is in every respect of 

 the same structure as the latter membrane. 



In the larger centres, the brain and spinal cord, the coverings are 

 of a more complicated kind. They are called meninges, membranes. 

 Three of them are enumerated : The dura mater, which is external ; 

 the pia mater, which is in immediate connexion with the nervous 

 matter of the centre; and the arachnoid membrane, a serous sac 

 intermediate to the two tunics just mentioned, which is evidently 

 destined to facilitate the movements of these organs within their pro- 

 per cavities. These will be more particularly described further on. 



In examining the ganglia, we obtain a good 

 idea of the minute structure of nervous centres 

 in general. A thin slice of one of the larger 

 ganglia, torn up by needles, or a small ganglion 

 from some small animal, serves to shew the dis- 

 position of the vesicular and fibrous matter in 

 these bodies. 



A ganglion may be compared to a plexus, 

 with nerve-vesicles deposited in its meshes (figs. 

 62, 63). In tracing a nerve into a ganglion, 

 its component fibres appear to separate, and 

 to pass through the ganglion in different direc- 

 tions; some maintaining their original course, 

 others diverging from it for a short way and 

 afterwards returning to it, and others taking 

 altogether a new direction and passing out of 

 the ganglion in combination with other fibres, 

 to form an emerging nerve. A certain degree 



Fi C2 



6 5 

 Second abdominal ganglion 



* 



