144 



ZOOLOGY. 



it first takes the path of least resistance : the resistance in 

 an artery is caused by the elasticity of its walls, conse- 

 quently the less the length of artery between a given point 

 and the capillaries, the less the resistance. It follows that 

 the resistance is least in the pulmo-cutaneous arches, and so 

 the blood first flows along these ; but as blood is pumped 

 into these and their walls are dilated the resistance increases, 

 and the blood begins to force itself along the synangium 

 into the systemic and carotid arches. As it does so it 

 forces over the longitudino-spiral valve and prevents 

 further flow into the pulmo-cutaneous arches. Its next 

 choice between systemic and carotid arches is determined in 

 favour of the former by the extra resistance which the 



carotid gland causes 

 in the latter. Thus 

 it comes about that 

 the first and most 

 deoxygenated blood 

 goes to the lungs and 

 skin, the last and 

 most oxygenated to 

 the head, while rather 

 less perfectly oxy- 

 genated blood goes 

 along the systemic 

 arch to all other 

 parts. 



7. The Arteries. 

 Fig. 70 shows the 

 general arrangement 

 of the arteries. In 

 comparison with the 

 rabbit we may specially 

 note that as there are 

 two systemic arches, so too there are two dorsal aortse, 

 which eventually unite in the middle dorsal line as the 

 median dorsal aorta. Just at the junction of the two 

 comes off a single median vessel, the ccdiaco-mesenteric, in 

 place of the two separate coeliac and anterior mesenteric 



Jig. 70. PRINCIPAL ARTERIES OF FROO. 



