PHYSIOLOGICAL 437 



through the walls of the blood-vessels to the seat of irritation or 

 injury; in Vertebrates this power is frequently illustrated. 



As the circulatory system becomes gradually more highly devel- 

 oped among Vertebrate or Chordate animals, from Tunicates 

 onward», the number of extra- vascular phagocytes is reduced, and 

 more and more work devolves on those of the blood. In the fin of 

 a young newt an injury or an infection may be dealt with solely 

 by the migratory phagocytes of the connective tissue ; in the more 

 frequently observed case — the tail of a tadpole in which blood-vessels 

 have been developed — the extra- vascular phagocytes are greatly 

 aided by leucocytes, which work their way through the walls of the 

 vessels, or are liberated, it may be, by a lesion; in a third set of 

 cases the whole process of defence or repair depends on the leuco- 

 cytes. It is important also to notice that the endothelial cells lining 

 the blood-vessels may by their contractility assist in the passage 

 of the leucocytes. Sometimes, moreover, as we shall see later, they 

 may free themselves from the wall of the vessel to deal more effec- 

 tively with bacteria that have been introduced into the blood. 



Metchnikoff's particular service was to show that most animals 

 have this bodyguard of free amoeboid cells or phagocytes, which 

 are useful in many ways, (i) They are sensitive to bacteria and other 

 intruders, and move towards them; they engulf and digest them. 

 They often crowd together in a clump, and this may lead to a walling 

 in of the microbes which prevents their spreading. It was believed 

 for a time that the phagocytes produced "opsonins", which make the 

 bacteria more susceptible to the attack; but this hypothesis has 

 given place to the view that ("adsorptive") changes in surface 

 tension affect the intruding bacteria, rendering them less resistant. 

 Adsorption means the decrease of free energy at the surface between 

 a solution and a body. The "opsonic index" of the blood is simply 

 the varying degree in which it affects the surface of bacteria that 

 have found their way in and are multiplying. 



It is not to be supposed that the bacteria always retreat when 

 the phagocytes appear on the scene. On the contrary, some of them 

 secrete products which keep the phagocytes at a distance; others 

 are able to envelop themselves in a protective capsule; and others, 

 unfortunately, are able to ferment the phagocyte that has engulfed 

 them. 



(2) Phagocytes also play a part in transporting material, both 

 nutritive and otherwise, from one area of the body to another. 

 According to Metchnikoff, the occasional rapid blanching of hair or 

 feathers is due to the activity of phagocytes, which transport 

 granules of the dark pigment (melanin) from the hair or feather 

 into the skin. 



(3) In the healing of wounds and in the replacement of lost parts, 

 such as occurs at the cut end of a worm, there is often phagocytic 



