RETE MIEABILE. 801 



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

 a Seal. 



(2342.) In many of the long-necked herbivorous quadrupeds a pecu- 

 liar 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 influxions to which it would otherwise be constantly 

 liable. 



(2343.) We must likewise notice a structure, in some respects similar 

 to the above, that exists in the arteries both of the anterior and pos- 

 terior extremities of the Sloth (Bradypus). In these slow-moving 

 animals, the axillary and iliac arteries, just before 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 multi- 

 plied 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. 



(2344.) Innumerable other minor differences in the course and dis- 

 tribution of the blood-vessels might of course be pointed out, a few 

 of which may require notice elsewhere ; but, generally speaking, the 

 arrangement of the vascular system in all quadrupeds is so similar, that 

 the anatomical student who may push his researches thus far will never 

 be at a loss in identifying the different vessels, and comparing them with 

 those found in the human body. 



(2345.) Although the respiration of Mammalia is inferior, as regards 

 the extent to which their blood is exposed to the influence of the atmo- 

 sphere, to the perfection of this process in Birds, nevertheless such is 

 the elevated temperature of the body in these hot-blooded animals, that 

 a warm covering of some non-conducting material is here absolutely 

 requisite to retain the vital warmth, and defend them against the ther- 

 mometrical changes of the element they inhabit. Their skin is gene- 

 rally, therefore, clothed with a warm covering of hair, a cuticular struc- 

 ture the nature and growth of which it behoves us now to examine. 

 We must first, however, notice the organization of the skin itself; and 

 then the nature of the various structures employed to defend it will be 

 readily understood. 



(2346.) The skin of all Mammals, like that of the human body, con- 



3F 



