TYPICAL STRUCTURE OF ORGANS 41 



nerve trunks being found in the septa, and their fine ultimate 

 branches being distributed by way of the connective tissue 

 which surrounds the cells, in whose neighborhood or even 

 within whose substance they finally end. As we shall see in 

 subsequent chapters, it is the function of the nerves to arouse 

 the gland cells or muscle fibers or other cells to activity. 



15. Summary. Disregarding for the moment those pecu- 

 liarities of arrangement, shape, and structure of the cells 

 which are connected with the special work of each organ 

 (for example, the arrangement of gland cells to form a blind 

 tube or of the connective tissue and fibers of muscle so as 

 to exert a pull on a bone), we may say that the typical 

 structure of an organ would be represented in Fig. 31. The 

 whole is surrounded by a capsule, receives a blood supply 

 through a system of closed tubes, and contains the special 

 cells ' upon whose activity its characteristic work depends. 

 These cells are held together by a fine connective tissue 

 whose numerous and freely communicating spaces contain 

 a fluid, the lymph, which is free to flow out through a 

 second system of tubes, the lymphatics. Nerves from the 

 brain or spinal cord are also distributed in the connective 

 tissue to the cells of the organ. 



Before concluding this description of the finer structure 

 of organs, a word may be added with regard to the physical 

 nature of the cell substance. In its literal meaning the word 

 "cell" is a misnomer, since it suggests a hollow space inclosed 

 by solid partitions or walls. Plant cells do, in fact, usually 

 have such walls around their cytoplasm (Chap. VIII), and 

 this cytoplasm frequently contains spaces (vacuoles) filled 

 with a solution of salts, sugar, and other dissolved material ; 

 but neither the cell wall nor vacuoles are of universal occur- 

 rence, each being rarely found in the animal cell, and often 

 absent even in the plant cell. Fifty years of thorough investi- 

 gation has reduced the number of essential cell constituents 

 to the cytoplasm and the nucleus, the ultimate structure of 



