VASCULAR SYSTEM OF FROG 665 



The blood contains in its fluid plasma — {a) the oval 

 " red " corpuscles, with a definite rind, a distinct nucleus, 

 and the pigment haemoglobin ; (b) white corpuscles or 

 leucocytes, like small amoebae in form and movements ; 

 (c) very minute bodies, usually colourless and variable in 

 shape. When the blood clots, the plasma becomes a 

 colourless serum, traversed by coagulated fibrin filaments, 

 the red corpuscles often arrange themselves in rows, and 

 the white corpuscles are entangled in the coagulated shreds. 

 When the web of a living frog is examined under the micro- 

 scope, it will be seen that the flow of blood is most rapid in 

 the arteries, more sluggish in the veins, most sluggish in the 

 capillaries or fine branches which connect the arteries and 

 the veins. The red corpuscles are swept along most 

 rapidly, and are often deformed by pressure ; the leucocytes 

 tend to chng to the walls of the capillaries, and may indeed 

 pass through them (diapedesis). 



The arterial system. — Each branch of the aortic trunk is 

 triple, and divides into three arches : — 



I. The carotid arch, the most anterior, corresponding 

 to the first efferent branchial of the tadpole, gives off — 



A lingual artery to the tongue ; 



A carotid artery, which bears near the origin of the lingual a 

 spongy swelling (the "carotid gland"), and gives off an 

 external carotid to the mouth and the orbit, and an internal 

 carotid to the brain. 



II. The systemic arch, the median one of the three, 



corresponding to the second efferent branchial in the 



tadpole, gives off — 



The laryngeal artery to the larynx ; 



The oesophageal to the oesophagus ; 



The occipito-vertebral to the head and vertebral column ; 



The subclavian or brachial to the fore^-limb. 



From the left aortic arch, just as it unites with its fellow 

 of the other side to form the dorsal aorta, or from the 

 beginning of the dorsal aorta, there is given off the coeliaco- 

 mesenteric to the stomach, intestine, liver, and spleen. 



Farther back the dorsal aorta gives off — 



The renal arteries to the kidneys, and the genital arteries to the 



reproductive organs ; 

 The inferior mesenteric to the large intestine. 



