PHYLUM CHORDATA 



403 



(Co). The skin is chiefly protective and sensory, but may also 

 carry on respiration and excretion. Excretion takes place by 

 means of glands, which may be simple, as the mucous glands of 

 fishes, or complex, as the sweat, oil, and mammary glands. The 

 skin often produces numerous 

 outgrowths such as hair, feathers, 

 nails, hoofs, claws, scales, teetti, 

 and bony plates. 



Skeleton. The outgrowths 

 of the integument noted above 

 constitute the exoskeleton. The 

 internal supporting framework 

 of the body is the endoskeleton. 

 This consists of (i) an axial 

 portion comprising the skull and 

 vertebral column, and (2) an 

 appendicular portion which sup- 

 ports the appendages. 



The bones of the endoskeleton 

 are typically formed in and 

 around cartilage. The animal 

 part of the bone is the cartilage; 



this can be obtained by dissolving out the mineral part, the bone- 

 ash, in hydrochloric acid. The bone-ash consists principally of 

 carbonate and phosphate of lime, and is the residue when a bone 

 is burned. The mineral constituents give the bone rigidity; 

 the cartilage furnishes plianc^ and elasticity. Bones support 

 the soft parts, furnish points of attachment for the muscles, and 

 protect certain delicate organs, such as the brain, spinal cord, 

 and eyes. 



The axial skeleton consists typically of the skull, the vertebrae, 

 and the ribs which may be attached to a ventral bone, the 

 sternum. The skull includes a brain case or cranium, which 

 protects the brain, and a -visceral skeleton, which supports the 

 respiratory apparatus and includes the facial bones. 



FIG. 347. Section through human 

 skin. Co, dermis ; F, subcutaneous 

 fat; GP, vascular papillae; H, hair with 

 sebaceous glands (D); N, G, nerves; 

 NP, sensory papillae ; Sc, stratum 

 corneum ; SD, sweat-glands with 

 their ducts (SD'); SM, stratum mal- 

 pighi. (From Wiedersheim.) 



