VERTEBEATE TYPE. 6 



brate type of organisation, although there is an agi-eement in the repetition of 

 parts of like structure in a longituduial series ; and it is rmnecessaiy here to 

 trace the correspondence between their structure and that of man. 



In the human body, as in that of all vertebrate animals, the character of 

 segmentation is most obvious in the osseous and nervous systems, so that the 

 form and structui-e of the other systems seem to be in some measure moulded 

 upon those of the skeleton and cerebro-spinal axis. 



Tlie trunk of the body more especially is formed of a series of parts or 

 segments of similar structure sufficiently distinct in some of the systems, but 

 more or less blended together in others. Such ideal segnients of the body may be 

 named rertcbral scr/ments, or aomatonws (Goodsir). In the limbs, although in the 

 earliest stages of their fonnation some segmental connection may be traced 

 between them and the trunk, the repetition of vertebral elements is, in their 

 more advanced state of growth greatly obsciured by the modifications of form 

 and structure they have undergone. 



"Vertebrate Homolog'y. — A correspondence in the structure and connection of 

 parts or organs constitutes what is now called anatomical liomology. and the 

 same term is frequently employed to designate anatomical correspondence of 

 parts serially repeated in. the same animal : but for this last or serial homo- 

 logy the modified term of homotijjxj, suggested by Owen, may with advantage 

 be substituted. Thus, the arm-bone or humerus of man is homologous with the 

 upper bone of the foreleg of a quadruped or of the wing of a bird, while it is 

 properly homotjqjic with the thigh-bone of man himself or any other vertebrate 

 animal. It has fm-ther been found convenient to express by the word analogy 

 that kind of resemblance among the organs of animals which, though indicating 

 similarity of function, and even in some respects of structiu-e. is not rendered 

 complete by anatomical relation and connection. This general resemblance may 

 be very great ; but it is of a different kind from that absolute identity which is 

 implied by the term anatomical homology, which is intended to convey the idea 

 of entire correspondence in fundamental structure, position, and connection of 

 any organ, or set of organs, in relation to a general plan or tyi^e of constraction 

 of the organism, and which might be almost looked upon as synonymous with 

 anatomical identity — a correspondence, too, which is conceived to be traceable 

 through all the modifications, however great, which the foi-m and structure of 

 the organs may have undei'gone in the course of their development. Thus, 

 the heart of a fish or amphibian, though very different in external form and 

 inteiTial arrangement, is strictly homologous with that of a mammal or of man ; 

 or, in other words, it is anatomically the corresponding organ. But the heart of 

 the cnistacean, though muscular in its structure, and fitted to propel the blood 

 through the system, and thus analo/jous to the heart of a vertebrate animal, is 

 not regarded as strictly homologous with it, because it differs in its anatomical 

 relation to the rest of the organism, and cannot be referred to the same place in 

 a general plan of structure. 



Symmetry of Form. — A remarkable regularity of form pervades the organisa- 

 tion 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 by a plane which passes from 

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

 the parts previously mentioned, correspond almost exactly with each other, ex- 

 cepting by their lateral transposition, — and the human body thus presents in a 

 marked manner the character of lateral symmetry. There is, however, a depar- 

 ture 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 gi'eat bloodvessels, the liver, spleen, and some other 

 viscera, which are therefore styled the non-symmetrical parts or viscera. 



But, while the parts on each side of the mesial plane are thus so far symmetrical, 

 the same correspondence does not hold between parts situated in the dorsal and 

 ventral regions, nor even between those placed in the cephalic and caudal extre- 

 mities of the body. In the fii-st, with the exception of the division of the 

 ventrally-placed stemimi into segments which often correspond to a certain 

 number of the dorsal vertebriB, no such dorso-ventral symmetry exists : and in the 



