THE CIRCULATION OF THE BLOOD. 267 



subjects, are all easy of observation, and after a little practice 

 without liability of causing those aberrations due to the atten- 

 tion being drawn to one's self. 



The observations need not, of course, be confined to the stu- 

 dent's own person ; it is, however, very important that the nor- 

 mal should be known before the observer is introduced to cases 

 of disease. Frequent comparison of the natural and the dis- 

 eased condition renders physiology, pathology, and clinical 

 medicine much good service. We again urge upon the student 

 to try to form increasingly vivid and correct mental pictures of 

 the circulation under its many changes. 



Comparative, — An interesting arrangement of blood-vessels, 

 known as a rete mirabile, occurs in every main group of verte- 



FiG. 221.— JB«fe miroMle of sheep, seen in profile (after Chaiiveau). The larger rete is 

 in connection with the encephalic arteries; the smaller, the ophthalmic. TThe large 

 artery is the carotid. 



brates. An artery breaks up iato a great number of vessels of 

 nearly the same size, which terminate, abruptly and without 

 capillaries, in another arterial trunk. 



They are found in a variety of situations, as on the carotid 

 and vertebrate arteries of animals that naturally feed from the 

 group.d for long periods together, as the ruminants ; in the 

 sloth, that hangs from trees ; in the legs of swans, geese, etc. ; 

 in the horse's foot, in which the arteries break up into many 

 small divisions. It has been suggested that these arrangements 



