INTRODUCTION. 21 



respectively, as the pleural and peritoneal membranes, which, in 

 addition, are reflected over the surfaces of the organs contained 

 within them. The alimentary canal and the various cavities con- 

 nected with it are lined throughout by a mucous membrane. 



The surface of the body is covered by the skin. This is com- 

 posed of an inner portion, the derma, and an outer portion, the epi- 

 dermis. The former consists of fibers, blood-vessels, nerves, etc. ; the 

 latter of layers of scales or cells. Embedded within the skin are 

 numbers of glands, which exude, in the different classes of animals, 

 sweat, oily matter, etc. Projecting from the surface of the skin are 

 hairs, bristles, feathers, claws. Beneath the skin are found muscles, 

 bones, blood-vessels, nerves, etc. 



The appendicular portion of the body consists of two pairs 

 of symmetric limbs, which project from the sides of the trunk, and 

 which bear a determinate relation to the vertebral column. They con- 

 sist fundamentally of bones, surrounded by muscles, blood-vessels, 

 nerves and lymphatics. The limbs, though having a common plan 

 of organization, are modified in form and adapted for prehension 

 and locomotion in accordance with the needs of the animal. 



Anatomic Systems. All the organs of the body which have 

 certain peculiarities of structure in common are classified by anato- 

 mists into systems e. g., the bones, collectively, constitute the bony 

 or osseous system; the muscles, the nerves, the skin, constitute, 

 respectively, the muscle, the nerve, and the tegumentary systems. 



Physiologic Apparatus. More important from a physiologic 

 point of view than a classification of organs based on similarities of 

 structure is the natural association of two or more organs acting 

 together for the accomplishment of some definite object, and to 

 which the term physiologic apparatus has been applied. While in 

 the community of organs which together constitute the animal body 

 each one performs some definite function, and the harmonious co- 

 operation of all is necessary to the life of the individual, everywhere 

 it is found that two or more organs, though performing totally dis- 

 tinct functions, are cooperating for the accomplishment of some 

 larger or compound function in which their individual functions are 

 blended e. g., the mouth, stomach, and intestines, with the glands 

 connected with them, constitute the digestive apparatus, the object 

 or function of which is the complete digestion of the food. The 

 capillary blood-vessels and lymphatic vessels of the body, and espe- 

 cially those in relation to the villi of the small intestine, constitute 

 the absorptive apparatus, the function of which is the introduction 

 of new material into the blood. The heart and blood-vessels con- 

 stitute the circulatory apparatus, the function of which is the dis- 

 tribution of blood to all portions of the body. The lungs and trachea, 



