6 DEFINITIONS. Sect. II. 2. 4. 



4. The mufcuhr fibres are moving organs intermixed with that 

 medullary fubftance, which is continued along the nerves, as 

 mentioned above. They are indued with the power of contrac- 

 tion, 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 ten- 

 dons, and thofe on the other fide of the fore-arm extend them 

 again. The arteries are diftended by the circulating blood ; 

 and in the necks of quadrupeds there is a ftrong elaftic liga- 

 ment, which afiifts the mufcles, which elevate the head, to keep 

 it in its horizontal pofition, and to raife it after it has been de- 

 preiTed. 



5. The immediate organs of fenfe confift in like manner of 

 moving fibres enveloped in the medullary fubftance above men- 

 tioned ; and are erroneoufly fuppofed to be fimply an expanfion 

 of the nervous medulla, as the retina of the eye, and the rete 

 mucofum of the fkin, which are the immediate organs of vifion, 

 and of touch. Hence when we fpeak of the contractions of the 

 fibrous parts of the body, v/e fhall mean both the contractions 

 of the mufcies, and thofe of the immediate organs of fenfe. 

 Thefe fibrous motions are thus diftinguifhed from the fenforial 

 motions above mentioned. 



6. The external organs of fenfe are the coverings of the im- 

 mediate organs of fenfe, and are mechanically adapted for the 

 reception or tranfmiflion.of peculiar bodies, or of their qualities, 

 as the cornea and humours of the eye, the tympanum of the ear, 

 the cuticle of the fingers and tongue. 



7. The word idea has various meanings in the writers of 

 metaphyfic : it is here ufed fimply for thofe notions of external 

 things, which our organs of fenfe bring us acquainted with orig- 

 inally ; and is defined a contraction, or motion, or configuration, 

 of the fibres, which conftitute the immediate organ of fenfe ; 

 which will be explained at large in another part of the work. 

 Synonymous with the word idea, we (hall fometimes ufe the 

 words fenfual motion in contradiftinction to mufcular motion. 



8. The word perception includes both the action of the or^an 

 of fenfe in confequence of the impact of external objects, and 

 our attention to that action ; that is, it exprefles both the motion 

 of the organ of fenfe, or idea, and the pain or plcafure that fuc- 

 ceedsor accompanies it. 



9. The pleafure or pain which necefiarily accompanies all 

 thofe perceptions or ideas which we attend to, either gradually 

 fubfides, or is fuccceded by other fibrous motions. In the latter 

 cafe it is termed Jen/at ion, as explained in Sect. V. 2, and VI. 



e reader is intreatcd to keep this in his mind, that 



through 



