686 MAMMALIA. 



the circulating blood during the temporary suspension of respira- 

 tion that occurs whenever the animal plunges beneath the surface 

 of the water. 



(785.) In other aquatic Mammals that dive, and are thus sub- 

 jected to prolonged immersion, large dilatations are found con- 

 nected with the principal trunks of the venous system in the 

 neighbourhood of the heart, in order to prevent a dangerous dis- 

 tension of these veins while the circulation is impeded and re- 

 spiration put a stop to. This is particularly remarkable in the Seal 

 tribe ; and in these Carnivora we are assured by good authorities 

 that it is not uncommon to find the foramen ovale of the heart, 

 and the ductus arttriosus, which in the fetus allows blood to pass 

 from the pulmonary artery directly to the aorta, still open even in 

 the adult animal ; but this arrangement, as we are well satisfied, 

 is by no means to be regarded as the normal structure of the heart 

 in a Seal. 



(786.) In many of the long-necked herbivorous quadrupeds a 

 peculiar provision has been made in the disposition of the internal 

 carotid arteries, apparently intended to equalize the force of the 

 blood supplied to the brain in different positions of the head: for 

 this purpose the arteries referred to, just as they enter the skull, 

 divide into several branches, which again unite so as to assume a 

 kind of plexiform arrangement, forming what is called the rete 

 mirabile of old authors. The effect of this subdivision of the 

 main trunk into so many smaller channels will evidently be to 

 moderate the rapidity with which the blood would otherwise enter 

 the cranium, and thus preserve the brain from those sudden in- 

 fluxions to which it would otherwise be constantly liable. 



(787.) We must likewise notice a structure, in some respects 

 similar to the above, that exists in the arteries both of the an- 

 terior and posterior extremities of the Sloths (Bradypus). In 

 these slow-moving animals, the axillary and iliac arteries, just be- 

 fore entering the limbs to which they are respectively destined, 

 suddenly divide into numerous small channels, which again unite 

 into one trunk before the arteries of the member are given off. 

 No doubt such an arrangement will very materially retard the 

 course of the blood as it flows through these multiplied canals, and 

 perhaps is materially connected with the long-enduring strength of 

 muscle that enables these creatures to cling without fatigue to 

 the branches whereby they suspend themselves. 



Innumerable other minor differences in the course and distribu- 



