On Muscular Motion and Animal Spirits 243 



on the other hand, animals indulging in ease and free 

 from hard work become very obese, and fat is de- 

 posited on their muscles in quite sufficient abundance. 

 Whence we may gather that the sulphureous par- 

 ticles of the blood, of which the fat is formed, have 

 some share in the production of muscular contraction. 

 Indeed, animals seem to be emaciated by hard work 

 just because the sulphureous and fat particles of the 

 blood are used up and consumed in muscular con- 

 traction, while yet if the motor parts have long ceased 

 from contraction the sulphureous particles are not now 

 spent in producing the contraction of the fibrils, 

 but are carried away through special vessels into 

 suitable receptacles and, brought there in sufficient 

 abundance, constitute fat. But that the fatty par- 

 ticles are carried by special vessels, is proved by the 

 fact that the blood-vessels disseminated through the 

 mass of the muscle do not extend to the fat connected 

 with almost all muscles, so that the sulphureous 

 particles composing the fat cannot come immediately 

 from the blood, but they must be brought by special 

 vessels from the inner part of the muscle ; and 

 certainly it has been established by anatomical obser- 

 vation that certain membranous vessels dispersed here 

 and there through the mass of the fat are continued 

 into the inner parts of the muscles. It does not 

 seem likely that the fat thus stored in appropriate 

 receptacles returns to the motor parts, still it is not to 

 be by any means regarded as a useless excrement, but 

 as serving various ends, such as the support of delicate 

 parts and the lubrication of others. But these things 

 will be discussed more at length later. In the 

 meantime let us inquire as to the nature of the 

 animal spirits, the influx of which into the muscles is 

 also necessary for bringing about their contraction. 



