MICROSCOPIC STUDY OF ANIMAL TISSUES. 55 



more is this the case in regard to the Teeth, whose organic struc- 

 ture (originally detected by Leeuwenhoek) has been newly and 

 far more completely elucidated by Profrs. Purkinje, Retzius, 

 Owen, and Tomes ; for the inquiry into the comparative struc- 

 ture of these organs, which has been prosecuted by Prof. Owen, 

 in particular, through the entire range of the Vertebrated series, 

 has shown that, with an equally close conformity to a certain 

 general plan of structure, there are at the same time still wider 

 diversities in detail, which are so characteristic of their respective 

 groups, that it is often possible to discriminate, not only families, 

 but even the genera and species, by careful attention to the 

 minute features of their structure. Similar inquiries, with 

 results in many respects analogous, have been carried out by the 

 Author, in regard to the Shells of Mollusks, Crustaceans, and 

 Echinoderms ; his researches having not only demonstrated the 

 regularly-organized structure of these protective envelopes (which 

 had been previously affirmed to be mere inorganic exudations, 

 presenting in many instances a crystalline texture), but having 

 shown that many natural groups are so distinctively character- 

 ized by the microscopic peculiarities they present, that the in- 

 spection of a minute fragment of Shell will often serve to deter- 

 mine, no less surely than in the case of bones and teeth, the 

 position of the animal of which it formed a part. The soft parts 

 of the Animal body, moreover, such as the cartilages which 

 cover the extremities of the bones and the ligaments which hold 

 them together at the joints, the muscles whose contraction deve- 

 lopes motion and the tendons which communicate that motion, 

 the nervous ganglia which generate nervous force and the nerve- 

 fibres which convey it, the skin which clothes the body and the 

 mucous and serous membranes which line its cavities, the assimi- 

 lating glands which make the blood and the secreting glands which 

 keep it in a state of purity, these, and many other tissues that 

 might be enumerated, are severally found to present character- 

 istic peculiarities of structure, which are more or less distinctly 

 recognizable throughout the Animal series, and which bear the 

 strongest testimony to the Unity of the Design in which they all 

 originated. As we descend to the lower forms of Animal life, 

 however, we find these distinctions less and less obvious ; and 

 we at last come to fabrics of such extreme simplicity and homogene- 

 ousness, that every part seems to resemble every other in struc- 

 ture and action ; no provision being made for that " division of 

 labor" which marks the higher types of organization, and which, 

 being the consequence of the development of separate organs 

 each having its special work to do, can only be effected where 

 there is a "differentiation" of parts, that gives to the entire 

 fabric a character of heterogeneousness. 



The Microscopic investigation whose nature has thus been 

 sketched, has not only been most fruitful in the discovery of 

 individual facts, but has led to certain general results, of great 



