498 Animal Biology 



form a secretory organ known as a gland. Glands may emit their secre- 

 tions through permanent channels, as in the case of the salivary glands, 

 or distribute them throughout the body by means of the blood stream and 

 tissue fluids. The latter type of gland is called a ductless (endocrine) 



gland. 



These are considered in detail later in this chapter. 



Superior rectus /A. 

 ' Medial rectus A/I. 

 1 / _...=:..^rr.??ii'-l>' _pauey 



Sup. oblique M' 



Lid 



/ Eyelash 



Iris 



Lateral 



rectus M. 



Eyelash 



Lid 



Lateral rectus AL \ "^^r^H^;^' ^)f j'^1 _ _ /nf. obi ique M. 

 Inferior rectus M. 



Fig. 249. — Human eye muscles and optic nerve. Observe that the muscles are 

 in pairs but that the superior oblique runs through a pulley and is longer than 

 the inferior oblique. 



semicircuTar 

 canals 



pharynx 



Fig. 250. — Human ear, in section, shown diagrammatically. The car bones 

 are malleus (hammer), incus (anvil), and stapes (stirrup). (By permission from 

 General Zoology by Storer. Copyright, 1943. McGraw-Hill Book Company, Inc.) 



The contraction of a muscle is accompanied by electrical, mechanical, 

 and thermal changes. Contraction may be caused by nerve impulses or 

 by chemical substances. When a muscle is stimulated there is a short 

 latent period of about .01 of a second, followed by a period of contraction 

 of about .04 of a second, then a period of relaxation (lengthening) of 



