41 INTRODUCTION. 



Those in which the nutritive juices, absorbed by 

 the coats of the intestines, are immediately dis- 

 persed through the porous regions of the whole 

 body are more numerous, inasmuch as the whole 

 class of insects appears to come under this de- 

 scription. 



But in some of these, including arachnoides and 

 worms, the nutritive fluid circulates through a 

 system of covered vessels, from the final branches 

 of which the nutritious particles are dispersed to 

 the several parts requiring their sustenance. The 

 vessels which thus convey the nutritive fluid are de- 

 nominated arteries, and those which return it to 

 the centre of the circulating system receive the ap- 

 pellation of veins. The circulating vortex, or round 

 in which the nutritive fluid travels, is sometimes 

 perfectly simple, sometimes it is two, and some- 

 times threefold, if we include the vena portarum. 

 The rapidity of its motion is often accelerated by 

 the contractions of certain fleshy preparations 

 termed hearts, situated at one or the other of the 

 two central points of the circulating system, and 

 occasionally at both. 



In the vertebrated red-blooded animals the nu- 

 tritive fluid issues from the intestines of a white 

 colour, and has then the name of chyle. It pro- 

 ceeds through certain vessels called lacteal into the 

 venous system, where it is mingled with the blood. 

 Vessels similar to the lacteal, and forming by their 

 assemblage what is termed the lymphatic system, 

 carry into the blood the remnant of the nutritious 



