ORGANS AND SYSTEMS 103 



7. Muscular System. — Motion and locomotion. 



8. Nervous System. — Reception of stimuli, sensation, coordination, 

 and causation of muscular and secretory activity. 



9. Reproductive System. — Reproduction. 



This enumeration does not cover all of the structures in the body. 

 Such a tissue as fat, the function of which is storage, does not logically 

 come under the head of any one of these systems. In the lower Metazoa 

 many functions may be carried on in part by single cells and in part by 

 tissues, and when finally organs and systems become clearly defined 

 there is a gradual increase in the complexity of both, reaching its highest 

 degree in the highest animals. 



125. Organs Belonging to Different Systems. — Systems should 

 always be analyzed in terms of the organs which compose them. The 

 tegumentary system includes the skin and the structures contained in it, 

 with the exception of sense organs, which are usually considered as part 

 of the nervous system, and the skin muscles, which are generally referred 

 to the muscular system. The skin occupies a somewhat equivocal 

 position. It may play a part in absorption and in respiration as well as 

 performing the functions already given. It has often been considered 

 a single widely spread organ with smaller organs imbedded in it, such as 

 various glands, and organs of attachment, like sucking discs. 



The more important regions of the digestive system in the vertebrates 

 are the mouth, pharynx, esophagus, stomach, small intestine, large 

 intestine, and rectum, which may be considered, in a general sense, a 

 series of tubular organs placed end to end making up the aHmentary 

 canal. Some of these also contain other accessory organs, such as tongue, 

 teeth, and certain glands. Other accessory organs lying outside the 

 canal, as salivary glands, liver, and pancreas, also belong to this system. 

 The organs of the circulatory system are the heart, blood vessels, lymph 

 nodes, and spleen. The respiratory system includes, in various animals, 

 gills, lungs, air passages, and tracheae, or breathing tubes, of insects. 

 The pharynx, and in some cases the mouth, may be considered as belong- 

 ing to this as well as to the digestive system. Nephridia in the lower 

 forms and kidneys in the higher, the bladder, and the tubes which convey 

 the urine are the organs of the excretory system. 



The muscles, individually, are the organs of the muscular system 

 (Fig. 40) ; they contain muscle tissue and, in addition, fibrous connective 

 tissue sheaths, tendons, and nerve endings. In a similar way bones and 

 cartilages are examples of organs of the skeletal system; a bone, besides 

 bone tissue, may contain marrow, a fibrous sheath, and cartilage, which 

 coats certain areas on the bone, forming smooth surfaces for articulation 

 with other bones (Fig. 40). One organ may be included within another 

 and become a part of it, as blood vessels and nerves in muscles and 

 bones. 



