FROM SINGLE CELLS TO MANY CELLS 



75 



are connected with one another so that the 

 entire organ functions as a unit, that is, as 

 a syncytium. This is an apparent advantage 

 because the nature of its job requires almost 

 continuous operation. Striations are present, 

 but the nuclei are located deep within the 

 fibers rather than at the surface as in the 

 skeletal muscle fibers. Cardiac muscle cells 

 are so closely connected with one another 

 that a single nerve impulse sets them all 

 contracting at once, thus executing a single 

 powerful contraction. This is obviously very 

 desirable in a pump such as the heart. 



Nerve tissue. This type of tissue is com- 

 posed of neurons, special conducting cells 

 that are found throughout the brains and 

 nerve cords of all animals that possess a 

 centralized nervous system ( Fig. 4-7 ) . The 

 nerve cell is composed of a cell body, which 

 contains the nucleus and surrounding cyto- 

 plasm. Extending out from the cell body 

 are threadlike fibers, consisting of numer- 

 ous dendrites which normally convey im- 

 pulses to the cell body and a single axon 

 which usually conducts impulses away from 

 the cell body. The cell body maintains the 

 nutrition of the entire neuron, and if it is 

 destroyed the fibers die. However, nerve 

 fibers severed from their cell body will 

 usually be replaced by new fibers growing 

 out from the cell body. 



Cell bodies are concentrated into masses, 

 the most conspicuous of which are in the 

 brain and in the spinal cord; other masses 

 called ganglia have special locations in the 

 body. The nerves that we see on dissection 

 are made up entirely of fibers, each of 

 which is insulated by a fatty sheath, the 

 myelinated sheath. These units go to make 

 up the complex nervous system which we 

 shall study in more detail in a later chapter. 



ORGANIZATION OF TISSUES 



INTO ORGANS AND 



ORGAN SYSTEMS 



In order to perform specific functions, 

 tissues must be in someway incorporated 



into organs, because by definition any struc- 

 ture which performs a given function is an 

 organ. Obviously, a single contractile cell 

 could be an organ under this general defi- 

 nition. However, in the usual, restricted 

 sense, an organ is a group of tissues assem- 

 bled for the purpose of performing a spe- 

 cific function. The small intestine, for ex- 

 ample, is an organ whose function is the 

 digestion and absorption of food. It is com- 

 posed of layers of different tissues — an 

 outer layer of epithelial tissue covers the 

 gut throughout its length; immediately in- 

 side this are two layers of muscle tissue, 

 then a layer of connective tissue, and finally 

 a thin layer, one cell thick, of lining epithe- 

 lium. All of these tissues perform specific 

 jobs in bringing about the greater function 

 of digestion and absorption of food. Even 

 so, the small intestine is not adequate to 

 complete the job of ingestion, digestion, 

 absorption, and egestion as a single organ. 

 This greater function involves a series of 

 organs, the mouth, teeth, esophagus, stom- 

 ach, small intestine, liver, pancreas, colon, 

 and anus. In other words, the entire job is 

 done by a system of organs. Likewise, cir- 

 culation, breathing, excretion, and indeed 

 all bodily functions are performed by dif- 

 ferent organ systems. The combined activi- 

 ties of all of the organ systems constitute 

 an organism, or an individual. This can be 

 relegated to the cellular level, as in the case 

 of an amoeba in which all of the activities 

 take place within a single cell. On the multi- 

 cellular level, tissues, organs, and organ 

 systems have been assembled to make up 

 an organism which functions as a unit, just 

 as the single cell functions as a unit. 



THE ORGANIZED ANIMAL 



As cells became organized into groups 

 they took on a definite relationship to one 

 another, conveying to the resulting animal 

 a particular shape that can be described in 

 terms of symmetry. Symmetry refers to the 

 arrangement of parts in relation to points. 



