SECT. II. 2. DEFINITIONS. n 



fes along the innumerable ramifications of the 

 nerves to the various mufcles and organs of fenfe. 

 In thefe it lays afide its coverings, and is intermixed 

 with the flender fibres, which conftitute thofe muf- 

 cles and organs of fenfe. Thus all r(iefe diftant ra- 

 mifications of the fenforium are united at one of 

 their extremities, that is, in the head and fpine ; 

 and thus theie central parts of the fenforium con- 

 ftitute a communication between all the organs of 

 fenfe and mufcles. 



3. A nerve is a continuation of the medullary fub- 

 ftance of the brain from the head or fpine towards 

 the other parts of the body, wrapped in its proper 

 membrane. 



4. The mufculdr fibres are moving organs inter- 

 mixed with that medullary fubftance which is con- 

 tinued along the nerves, as mentioned above. They 

 are indued with the power of contraction, and are 

 again elongated either by antagonift mufcles, by 

 circulating fluids, or by elaftic ligaments. So the 

 mufcles on one fide of the fore-arm bend the fingers 

 by means of their tendons, and thofe on the other 

 fide of the fore-arm extend them again. The arte- 

 ries are diftended by the circulating blood ; and in 

 the necks of quadrupeds there is a ftrong elaftic li-, 

 gament, which aflifts the mufcles, which elevate the 

 head, to keep it in its horizontal pofition, and to 

 raife it after it has been depreffed. 



5. The immediate organs of fenfe confift in like 

 manner of moving fibres enveloped in the medul- 

 lary fubftance above mentioned ; and are erroneouf- 

 ly fuppofed to be fimply an expanfion of the ner- 

 vous medulla, as the retina of the eye, and the 

 rete mucofum of thefkin, which are the immediate 

 organs of viiion, and of touch. Hence when \ve 

 fpeak of the contractions of the fibrous parts of the 

 body, we fhall mean both the contractions of the 



mufcles. 



