VASCULAR SYSTEM OF THE FROG. 455 



purest blood to the head, and somewhat mixed blood to 

 the body. 



The blood contains in its fluid plasma — {a) yellowish 

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

 and the pigment haemoglobin, (1^) white corpuscles or leuco- 

 cytes, like small amoebEB 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 cor- 

 puscles 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 slugglish 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 

 cling to the walls of the capillaries, and may indeed pass 

 through them. 



The arterial system. Each aortic branch is triple, and 

 divides into the following on each side : — 



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 to the fore-limb. 



