524 



THE CELL AND MAMMALIAN HISTOLOGY 



any nerve supply, but its control in the adult, like that of smooth 

 muscle, is by double innervation. Since it is a syncytium, the 

 whole heart must act at once, within the limits of the time of 

 transmission of an impulse. 



BLOOD 



Thirdly and lastly we have the tissues which are based on 

 the amoebocyte. They correspond fairly accurately to what are 

 also called the fluids of the body because the ground substance 

 is liquid, not solid. It is obvious that an amcebocyte can only show 

 its properties fully in a liquid medium. First and most important 



A 



B 



V . . . . y 



C 



D 



':•'.'•. 



'm 



r 



O 

 o 



Fig. 410. — Blood Cells. — Adapted from Whitby and Britton. 



A and B, Polymorphonuclears ; C, monocyte ; D, small lymphocyte ; E, erythrocyte, in face and 

 edge view ; the shading is conventional and represents the hollow in the surface ; F, platelets, x 1000. 



is the blood. The liquid matrix is called plasma, and is an aqueous 

 solution containing the ions of sodium, potassium, calcium, 

 magnesium, and carbonate, together with glucose and other 

 organic compounds, and a protein called fibrinogen, which together 

 depress its freezing-point by 0.6° C. It is buffered, having in the 

 arteries a pH of 7.3 to 7.45. When blood is shed, or the tissues are 

 damaged, an enzyme thrombin is formed which changes fibrinogen 

 to threads of insoluble fibrin. These form a network in which the 

 cells become entangled, so that a clot is formed and bleeding 

 stops. When drawn blood is clotted there is left a clear liquid 

 which is plasma without its fibrinogen ; it is called serum. 



Blood is coloured red by the pigment haemoglobin, which is 

 entirely confined within small bodies called erythrocytes, red 

 blood corpuscles, or simply red cells. Each, in man (Fig. 410), 



