4 INTRODUCTION, 



become greatly obscured. To the original segments in the embryo the terms protovertebrce. 

 mesoblastic somites or myotomes have been applied ; those segments or metameres which are 

 traceable in the adult are often spoken of as vertebral segments. In the limbs, although there 

 is strong reason for believing that they have originated as outgrowths of certain segments of 

 the trunk, the repetition of such vertebral elements, and their primitive connection, are 

 greatly obscured. 



Homology. A certain agreement in structure, situation and connection of 

 parts or organs constitutes what is called homology, and this term is generally 

 employed to indicate the morphological identity of representative parts in different 

 animals, which may be considered to have its cause in community of origin (homo- 

 ffeny, Lankester), while the anatomical correspondence of parts which are repeated 

 in the same animal may be more exactly distinguished as serial homology (homo- 

 dynamy, Gegenbaur). Thus the arm-bone or humerus of a man is homologous 

 (homogenetic) with the upper bone of the fore limb of a quadruped, or of the wing 

 of a bird, while it is at the same time serially homologous (homodynamic) with the 

 thigh bone of man himself, or any other vertebrate animal. It has farther beer, 

 found convenient to express by the word analogy that kind of resemblance among 

 the organs of animals which depends upon similarity of function, and although it 

 may be accompanied by considerable agreement in structure, yet is not rendered 

 complete by anatomical relation and connection : for example, the gills of a fish, of 

 a crab, and of a mussel, serving the same function, are analogous organs, but in no 

 sense homologous, as all morphological correspondence, or genetic relation, is wanting 

 between them. Thus also, the upper limb of a man, the fore limb of a quadruped, 

 the wing of a bird, and the pectoral fin of a fish are homologous but not analogous 

 structures, the wing of a bat and the wing of a bird are both homologous and 

 analogous, while the last is analogous to but not homologous with the wing of an 

 insect. 



Symmetry of form. A remarkable regularity of form pervades the organi- 

 zation of certain parts of the body, especially the whole of the limbs, the head and 

 neck, and the framework, at least, and external walls of the trunk of the body. 

 Thus, if we conceive the body to be divided equally by a plane which passes from its 

 dorsal to its ventral aspect (median plane) , the two halves, in so far as regards the 

 parts previously mentioned, correspond almost exactly with each other, excepting by 

 their lateral transposition, and the human body thus shows in a marked manner 

 the character of bilateral symmetry. There is, however, a departure from this 

 symmetrical form in the developed condition of certain of the internal organs, such 

 as the alimentary canal from the stomach downwards, the heart and first part of the 

 great blood-vessels, the liver, spleen, and some other viscera. 



Descriptive terms. In the description of parts so numerous, so various in 

 form, and so complex in their connections as those composing the human body, there 

 is difficulty in finding terms which shall indicate with sufficient precision their 

 actual position and their relation to the rest of the organism. This difficulty is 

 farther increased by the exceptional erect attitude in which the trunk of the human 

 body is placed as compared with the horizontal position in animals. Hence, a 

 number of terms have long been in use in human anatomy which are understood in 

 a technical or restricted sense. For example, the median plane, already referred to, 

 being that by which the body might be divided into right and left lateral halves, 

 and the middle or median line being that in which the median plane meets the surface 

 of the body, the words internal and external are used to denote relative nearness to 

 and distance from this plane on either side, and may be replaced by mesial and 

 lateral. The terms sagittal, frontal, and coronal, are also used in indication of 

 direction within the body : sagittal denoting a dorso-ventral direction in or parallel 

 to the median plane, frontal or coronal a transverse direction perpendicular to that 



