6 INTRODUCTIOM. 



Such of, these as we have to allude to belong either to the natural 

 order Rasores or to the Natatores, the former including the 

 terrestrial birds and pigeons, the latter, the various aquatic species, 

 distinguished by their iveb feet. 



DESCRIPTIVE ANATOMY. 



Osteology is a term applied to that section of descriptive 

 anatomy which treats of the bones, Arthrology to the considera- 

 tion of the joints or modes of union between the bones, while by 

 Myology is meant the doctrine of the muscular system. Splanch- 

 nology treats of the viscera, Angiology of the circulatory and 

 absorbent systems. Neurology deals with the nervous system, 

 jEsthesiology with the organs of sense ; while Enibryology, as 

 before stated, is the consideration of the animal fraiue at periods 

 preceding its birth. In this work the various departments are 

 discussed in the order here given. The structures which are the 

 subjects of the first three divisions are sometimes classed together 

 as the Organs of Locomotion, for bones form the framework 

 of the body and often act as levers ; the joints connect the 

 bones, permitting more or less motion between them ; while the 

 muscles move the bones, and so produce motion of a part of the 

 body — or it may be locomotion, or change of situation in the 

 entire frame. 



In the study of Comparative Anatomy the terms analogy and 

 homology are frequently met with. Although these words are 

 not unfrequently used indiscriminately, the following difference 

 should be noted. Organs are said to be analogous when, though 

 differing in structure, they perform the same function ; but when 

 their functions are different, while, in the broad sense, they 

 correspond in structure or form, they are said to be homologous. 

 Thus the middle finger of the human hand is the homologue of 

 the anterior digit of a horse, because they have the same general 

 structure, and relation to the rest of the limb; but as the 

 functions they perform are quite dissimilar, they cannot be 

 termed analogous. Again, the lungs of a mammal are analogous 

 to the gills of a fish, for, though they differ widely in structure, 

 position, and form, and are therefore not homologous, their 

 ultimate use is the same — each of them being an apparatus in 

 which is carried on the process of purifying the blood. 



