34 



*** PHYSIOLOGY. 



TISSUES CONSOLIDATED BY EARTHY DEPOSIT.— BONES AND TEETH. 



Bone'Tee^h ^ description of these tissues, see Anatomy. Articles 



SIMPLE TDBULAR TISSUES. 



Of these we have examples in the smaller capillary vessels, and 

 probably also m the smallest lymphatics and lacteals. They seem 

 to be formed by the coalescence of the cavities of the cells, produced 

 by the absorption of the cell-walls at the point of contact 



In all the higher animals, in their adult condition at least, the ca- 

 piUary circulation is carried on through tubes having distinct mem- 

 branous walls. These tubes are formed from cells, ifke the straiaht 

 and anastomosing ducts of plants. In the walls of these tubes cell- 

 nuclei may be constantly found; and these are too far apart to war- 

 rant the idea, that they are the nuclei of epithelial cells, such as line 

 the larger vessels. 



n.T^T.r'fu"-^^'^-^ ^ ""^^'"^ ^^ ^^ ^'^g^^'^^d ^^^^"g the elementary 

 parts of the fabric, smce they are formed independently of the larger 

 trunks, and have little in common with them in function. All those 

 changes which take place between the blood and surroundina parts, 

 by which nutrition, secretion, and respiration are accomplished, occur 

 during the movement of the blood through these vessels, the laraer 

 vessels merely bringing to them a constant supply of fresh bloSd, 

 and conveying from them that which has been impoverished in the 

 loregoing processes. 



The diameter of the capillaries varies in different animals accord- 

 ing to the size of the blood-corpuscles. Thus, they are larger in the 

 trog than m man, in whom they average from l-3700th to ^l -2500th 

 ot an inch. In the living subject, it may be stated, they may be 

 seen to vary their diameter at different times, so as to accommodate 

 themselves to the varying supply of blood. They seem to have a 

 distributive power of their own, entirely independent of the heart's 

 action but influenced by the attraction existing between the tissues 

 and the constituents of the blood. The capillary vessels are first 

 lormed in the vascular layer of the germinal membrane, entirely by 

 the coalescence of cells, which send off prolongations in various 

 directions, like the radii from a star. By the junction of these pro- 

 longations a network of tubes is formed, at first, irregular in size, 

 but atterwards becoming more equalized. 



In newly.forming tissues, much the same arrangement takes place, 

 the prolongations coming in contact with the vessels of the surround- 

 ing parts. 



The opinion, that the lohile tissues are nourished by vessels con- 

 veying white blood is no longer tenable. Some of the white tissues, 

 such as cartilage, are entirely destitute of vessels; and in others the 

 supply of blood is so scanty, as not to communicate to them any 

 decided hue. What have been considered as white vessels, are 



