a 



38 ELEMENTARY STRUCTURE OF ORGAKIZED EODIE: 



the pith of the elder. They then 

 have the form of a honey-comb ; 

 whence they have derived their 

 name of celkdes. 



38. All the organic tissues, whether 

 animal or veG;etable, orijrinate from 

 cells. The cell is to the organ- Fig. 2. 



ized body what the primary form of the crystal is to tho 



secondary, in minerals. As a general fact, it muy be stated 



that animal cells are smaller than vegetable 



J,, cells ; but they alike contain a central dot or 



_ ^ vesicle, called nucleus. Hence such cells are 



called nucleated cells, (Fify. 8, a.) Sometimes 



°' ' the nucleus itself contains a still smaller 



dot, called nucleolus, (b.) 



39. The elementary structure of vegetables may be ob 

 served in every part of a plant, and its cellular character 

 has been long known. But with the animal tissues there is 

 far greater difficulty. Their variations are so great, and 

 their transformations so diverse, that after the embryonic 

 period it is sometimes impossible, even by the closest exam- 

 ination, to detect their original cellular structure. 



40. Several kinds of tissues have been designated in the 

 animal structure ; but their differences are not always well 

 marked, and they pass into each other by insensible shades. 

 Their modifications are still the subject of investigation, and 

 we refer only to the most important distinctions. 



41. The cireolar tissue consists of a network of delicate 

 fibres, intricately interwoven so as to leave numberless 

 communicating interstices, filled with fluid. It is inter- 

 posed in layers of various thickness, between all parts of 

 the body, and frequently accompanied by clusters of fat 

 cells. The fi'^^ous and the serous membranes are mere 

 •nodifications of this tissue. 



42 The cartilaginous tissue is composed of rucleated 



