ELEMENTABY STEUCTU11E OF ORGANIZED BODIES. 11 



crowd each other, their outlines become angular, and the 

 intercellular spaces disappear, as seen in figure 2, which repre- 

 sents the pith of the elder. They 

 then have the form of a honey-comb, 

 whence they have derived their name 

 of cellules. 



38. All organic tissues, whe- 

 ther animal or vegetable, originate 

 from cells. The cell is to the or- 

 ganized body what the primary form 

 of the crystal is to the secondary in 

 minerals. As a general fact, it may be stated that animal 

 cells are smaller than vegetable cells, but they alike contain a 

 central dot or vesicle, called the nucleus. Hence 'such cells 

 are called nucleated cells (Figs. 3 and 48). Sometimes the 

 nucleus itself contains a still smaller dot, called the nucleolus. 



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 examination, 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. 1 st. The areolar tissue consists of a network of deli- 

 cate fibres intricately interwoven, so as to leave numberless 

 communicating interstices filled with fluid. It is interposed, 

 in layers of various thickness, between all parts of the body, 

 and frequently accompanied by clusters of fat cells. The 

 fibrous and the serous membranes are mere modifications of 

 this tissue. 



have leaves, petals, stamens, pistils, roots, &c., all of which are indispen- 

 sable to the maintenance of life, and the perpetuation of the species. Since 

 the discovery of the fundamental identity of structure of animal and vege- 

 table tissues, a common denomination for this uniformity of texture has 

 been justly preferred ; and the existence of vital tissues is now regarded as 

 the basis of organization. 



