366 PRINCIPLES OF GENERAL PHYSIOLOGY 



taken up by pseudopodial processes of the phagocytes, since in the circulating blood 

 current they are spherical cells ; when a bacterium comes into contact with a 

 phagocyte, it is engulfed, killed, and digested. It will be clear that any influence 

 which makes adhesion more certain on chance contact will increase the number of 

 bacteria taken up, as will also, as Ledingham points out, any agglutination of the 

 bacteria into clumps, since a large number will be taken up at a chance encounter 

 instead of a single one. There does not seem to be any necessity for the assumption 

 of such ill-defined chemical substances as "opsonins," which have been supposed to 

 make the bacteria attractive or " tasty " to the phagocytes. 



The process of phagocytosis is also met with in the absorption of larval organs, 

 such as the tail of the tadpole. It plays a part in the formation of bone ; and 

 certain cells in the liver, Kupffer's "star-cells," are phagocytes. In the extra- 

 ordinary " disruption " of internal organs that takes place in the metamorphosis of 

 the fly, wherein nearly the whole of these organs become broken up preparatory to 

 the formation of the new organs of the adult, phagocytes play a considerable part. 



Digestion in the Sea Anemone. The process here deserves a little consideration, 

 since it forms a kind of transition to that of the higher multicellular animals. 

 Although there is what seems like a gastric cavity, no one has yet succeeded in 

 showing the presence therein of any secretion with digestive properties. Neverthe- 

 less, animals of considerable size are actually digested by the anemone and in 

 fact by a somewhat remarkable process. There are long filaments in great 

 number attached to the septa, in this gastric cavity and these wind themselves 

 all over the body of the animal taken in and into all depressions and cavities in it. 

 This they do by a kind of pseudopodial movement, probably conditioned by 

 surface tension, like that of the Amoeba. Where the cells covering the filaments 

 are in contact with food material, they secrete digestive enzymes limited to the 

 area of contact. These enzymes cause the tissues attacked to break up and 

 the fragments are then taken in by the cells and dealt with further by the 

 ordinary process of intracellular digestion. It appears that the enzymes can 

 diffuse for a short distance, since food wrapped up in filter paper is digested, 

 provided that the paper is moistened with meat extract. The presence of some 

 chemical stimulant in the meat causes the secretion of enzymes in the cells of the 

 mesenterial filaments at the places where they come into contact with it. 



GENERAL PLAN IN THE HIGHER ANIMALS 



The alimentary canal may be said to consist of a long tube, with dilatations in 

 places. In this tube the food is subjected to the action of a series of enzymes. 

 The dilated sections are capable of being shut off from the neighbouring sections 

 by means of rings of muscle, the sphincters, so that the food shall not be passed on 

 to the next section prematurely. As a rule, arrangements exist by which the 

 secretion of the digestive juice in a particular section is brought about by the 

 presence of food in the preceding section, so that no delay in the process occurs. 

 The food is kept in movement by muscular contractions of the walls of the canal 

 and passed on by similar movements. These movements are partly provided for 

 by nerve centres in the wall of the canal itself, but are under the control of the 

 central nervous system. As regards the sphincters, it is usual to find that they 

 close by a nervous reflex when a certain amount of material has been passed 

 through to the next section. 



Each of these factors requires a little more detail. In the following 

 description the state of affairs as fully developed in the case of one of the higher 

 vertebrates, man or the dog, is taken as typical ; in lower animals, vertebrate and 

 invertebrate, various simplifications are to be found, as well as special additions 

 for particular purposes. 



MOVEMENTS 



If the small intestine is cut out of an animal such as the rabbit, cat, or dog, 

 and immersed in warm, oxygenated Ringer's solution, or better, in Ringer-Tyrode's 

 solution, it is seen to exhibit a series of rhythmic contractions, which travel as 



