THE CONNECTIVE TISSUES. 



107 



such membranes are composed of flat, ribbon-like fibers or bands of 

 elastic tissue arranged in the form of a network, with larger or smaller 

 openings ; thus the term fenestrated membranes. F. P. Mall has 

 reached the conclusion that such membranes are composed of three 

 layers an upper and a lower thin transparent layer in which no 

 openings are found and which are identical with the sheaths of 

 elastic fibers described by this observer, and a central layer, contain- 

 ing openings, and staining deeply in magenta. This substance is 

 identical with the central substance of elastic fibers. 



Areolar con- 

 nective tis- 

 sue. 



Nucleus of con- 

 nective-tissue 



cell. 



Fig' 73- Tendon cells from the 

 tail of a rat. Stained in methylene- 

 blue (infra vitam}. 



74- Cross-section of ligamentum 

 nuchse of ox. 



4. ADIPOSE TISSUE. 



In certain well-defined regions of the body occur typical groups 

 of fixed connective -tissue cells which always change into fat-cells (fat 

 organs,Toldt). Connective -tissue cells in various other portions of the 

 body may also change into fat-cells, but in this case the fat, as such, 

 sometimes disappears, allowing the cells to resume their original con- 

 nective-tissue type, only again to appear and a second time change the 

 character of the tissue. The formation of fat is very gradual. Very 

 fine fat globules are deposited in the cell ; these coalesce to form 

 larger ones, until finally the cell is almost entirely filled with a 

 large globule (ind. also H. Rabl, 96). 

 As the fat globule grows larger and 

 larger, the protoplasm of the cell, to- 

 gether with its nucleus, is crowded to 

 the periphery. The protoplasm then 

 appears as a thin layer just within the 

 clear cellular membrane. The nucleus 

 becomes flattened by pressure, until 

 in profile view it has the appearance 

 of a long, flat body. In regions in which large masses of fat- 

 cells are developed, they are seen to be gathered into rounded 

 groups of various sizes (fat lobules) separated by strands of con- 

 nective tissue. Such lobules have, as was first pointed out by 

 Toldt, a typical and very rich blood-supply from the time that they 

 are recognized as fat organs in the embryo. A small artery 



rr.'.v. Nucleus. 

 Protoplasm. 



Fat drop. 

 Cell-membrane. 



Fig. 75' Scheme of a fat-cell. 



