52 FATHERS OF BIOLOGY. 



anatomy at which he especially worked. Indeed, he 

 seems to have had but few opportunities of carrying 

 on human dissections, for he thinks himself happy in 

 having been able to examine at Alexandria two human 

 skeletons ; and he recommends the dissection of monkeys 

 because of their exact resemblance to man. To this dis- 

 advantage may, perhaps, be attributed the readiness, which 

 sometimes appears, to assume identity of organization 

 between man and the brutes. Thus, because in certain 

 animals he found a double biliary duct, he concluded 

 the same to be the case in man, and in one instance he 

 proceeded to deduce the cause of disease from this 

 erroneous assumption. 



He supposed that there were three modes of existence 

 in man, namely 



(a) The nutritive, which was common to all animals 



and plants, of which the liver was the source. 



(b) The vital, of which the heart was the source. 



(c) The rational, of which the brain was the source. 

 Again, he considered that the animal economy pos- 

 sessed four natural powers 



(1) The attractive. 



(2) The alterative or assimilative. 



(3) The retentive or digestive. 



(4) The expulsive. 



Like his predecessors, he asserted that there were 

 four humours, namely, blood, yellow bile, black bile, and 



