58 



ESSENTIALS OF ZOOLOGY 



are the two common vascular tissues. Lymph has no red corpuscles. 

 In the blood of mammals the red corpuscles are without nuclei; 

 while in fish, frogs, turtles, and birds these cells are nucleated. The 

 chief function of this tissue is the transportation of digested food 

 and oxygen to the cells of the body and the removal of waste by- 

 products of metabolism from them. 



An organ is an arrangement of two or more tissues as a part of 

 the body which performs some specific function or functions. Some 

 organs are made up of all of the different types of tissues just de- 

 scribed. For example the stomach is an organ with an internal 



Fig. 15. Typical somatic cells from vertebrate animals. 1, Squamous epithelial 



cells- 2 section through a portion of bone showing Haversian canal (in center), 

 bone'cells, lacunae, canaliculi, and matrix; S, section of hyaline cartilage showing 

 cartilage cells in lacunae, and matrix between lacunae ; .',, section of tendon which 

 is composed of white fibrous connective tissue; 5, longitudinal view of smooth (m- 

 voluntary) muscle cells; 6, striated (voluntary) muscle; 7, motor nerve cell, show- 

 ing processes; 8, human red blood corpuscles (nonnucleated) and human white 

 corpuscles (nucleated). (Drawn by Titus C. Evans.) 



cavity. It is covered and lined with epithelium ; the wall contains 

 two strong layers of muscular tissue ; blood vessels carrying blood, 

 and lymph spaces bearing lymph, branch through the wall ; nervous 

 tissue reaches all parts of the organ to receive stimuli and distribute 

 impulses; and connective tissue serves to bind all the others in 

 proper relation. 



