xviii Contents 



full of nitro-aerial particles, p. 216. And the same of the 

 seminal liquors of the ^%%^ p. 216. How the blood of the 

 embryo is impregnated with aerial spirit in the umbilical 

 vessels, p. 217. The necessity of the umbilical arteries is 

 proved, p. 217. Of the respiration of the chick in the ^%%^ 

 p. 218. It is carried on by means of the umbilical vessels, 

 p. 218. How that takes place, p. 218. The warmth excited 

 in the ^g% seems to contribute towards taking the place of 

 respiration, p. 220. How the foetus just born and still 

 enclosed in its membranes can live without respiration, p. 

 221. On the cavity situated at the blunter end of every ^%%^ 

 p. 222. The air contained in it does not contribute to the 

 respiration of the chick, p. 223. That air is conspicuously 

 elastic, p. 223. By incubation the liquors of the ^gg 

 come to occupy a smaller space than they did before, p. 224. 

 In how many ways the .condensation of bodies may take 

 place, p. 224. How the liquors of the ^%% are condensed, p. 

 225. On the use of the air contained in the q%%^ pp. 226, 

 227, 



FOURTH TREATISE.— O^ MUSCULAR 

 MOTION. 



CHAPTER I.— Examination OF the Various Opinions 

 OF Authors as to the way in which Muscles 

 Contract. 



The nitro-aerial spirit contributes something to animal 

 motion, p. 229. The opinion of Dr Willis as to muscular 

 contraction is discussed, p. 230. And criticised, p. 231. 

 The contracted muscle draws itself together, p. 231. Dr 

 Steno's opinion as to the cause of muscular contraction is 

 examined, p. 233. The influx of new matter is required for 

 the contraction of muscles, p. 234. A muscle can be 

 shortened by a change of its shape without contraction of 

 its fibres, p. 234. 



