2 OSTEOLOGY. 



Correspondence of structural elements in different animals, or in different segments 

 of the same animal, constitutes what is now generally called liomology. Thus the 

 wings of birds and the fore limbs of quadrupeds are homologous with the upper limbs 

 of man, and vertebrae are homologous one with another. To distinguish the 

 correspondence of parts which lie in series from that which exists between struc- 

 tures in different animals, Owen has suggested for the former the term homotypy, 

 and the phrase serial liomology is likewise employed. Thus the bones of the foot 

 are serially homologous, or homotypic, with those of the hand. 



To express resemblance of a general nature, arising from similarity of function, the 

 term analogy has been adopted by recent authors. While structures fundamentally 

 similar are regarded as homologous, although the progress of their development 

 and the purposes for which they are adapted may be different, organs adapted for 

 similar purposes are said to be analogous, however different in their original 

 anatomical relations may be the parts of which they are composed. Thus the gills, 

 being the respiratory organs of fishes, are analogous to the lungs of the air-breathing 

 vertebrata ; but they are not homologous parts. 



DESCRIPTIVE TERMS. As it is the office of descriptive anatomy to indicate with 

 precision not only the form and structure, but also the position in relation to other 

 parts of objects which are often irregular and complicated, it is important that it 

 should have a generally recognised system of nomenclature : hence many words, 

 expressive of relative position, have acquired a restricted technical sense in which 

 they are generally used by anatomists. It may be proper to mention here the most 

 important of those terms. The mesial plane is that in which the body might be 

 divided into a right and left lateral half; the middle line being the line before and 

 behind, in which that plane meets the surface of the body. Internal and external 

 denote relative nearness to and distance from the mesial plane towards either side. 

 Superficial and deep indicate distance from the surface; and in the expression of this 

 relation it is well to avoid the use of the terms external and internal, inner and 

 outer, above and below, or to employ them with the utmost caution, lest they become 

 a source of confusion. Nevertheless, an exception to this rule is still very generally 

 made in the instances of some phrases which have been long in use. Thus the 

 superficial and deep oblique muscles of the abdomen are commonly called external 

 and internal, and so also with the superficial and deep inguinal rings. Superior, 

 inferior, above, below, anterior, and posterior have reference to the position of parts 

 in the erect posture of the body. As, however, the employment of those terms, 

 though unobjectionable in human anatomy, is liable to give rise to ambiguity in 

 general or comparative descriptions of structure in man and animals, it is frequently 

 preferable to substitute others which express position by reference to a region or 

 structure towards which the part is directed. Thus ventral and dorsal may be often 

 employed instead of anterior and posterior, and, in the limbs, proximal and distal 

 instead of superior and inferior. The inferior aspect of the lung may be called 

 diaphragmatic, the posterior aspect of the gullet vertebral, and so on. 



SECTION I. OSTEOLOGY. 



THE SKELETON. 



The osseous system forms the skeleton or solid framework of the body. 

 It supports the soft parts, protects delicate organs, and furnishes places of 

 attachment to the muscles, by winch the different movements are executed. 

 The osseous parts of the skeleton are bound together by ligaments ; and in 

 some parts the framework is completed by the addition of cartilages. A 

 natural skeleton is one in which the connecting parts have been preserved ; 

 while, in an artificial skeleton, the ligaments and cartilages having been 

 removed by complete maceration, the bones are united by artificial mechanical 

 contrivances, and the cartilages are replaced by some other substance. 



The number of bones in the skeleton varies at different ages of life, some 



