64 THE FROG 



prehallux. The first of the long digits on the preaxial side 

 corresponds to the great toe of man, and is called the hallux. 

 Each of the long digits consists of a metatarsal bone, and 

 phalanges forming the toes as follows : the first and second 

 toes have two phalanges each, the third three, the fourth 

 four, and the fifth three. 



Draw (X 2) the foot including the ankle. 



Make a table showing the homologies between the ele- 

 ments of the fore-limbs and the hind-limbs. 



XII. THE MUSCULAR SYSTEM. 



The movements of the frog are effected by means of 

 muscles. The distinguishing property of muscular tissue is 

 a high degree of contractility: i. e., the muscle fiber 

 shortens, thereby bringing the two ends, and consequently 

 the parts to which it is attached, nearer together. 



A. The Finer Structure of the Muscular System. 

 Muscular tissue may be classified according to the structure 

 of its cells into two kinds, striated and non-striated. The 

 former presents two varieties : (a) a kind forming the skeletal 

 muscles, which consist of unbranched striated cells each con- 

 taining many nuclei; (b) another kind, found in the heart, 

 is called cardiac muscular tissue, and consists of striated 

 cells which branch, and contain each a single nucleus. The 

 non-striated muscle cells are usually unbranched, are lack- 

 ing in cross-striations, and contain each a single nucleus. 



With regard to nervous control, muscular tissue may be 

 classified as voluntary and involuntary. The voluntary 

 muscle fibers are under the immediate control of the central 

 nervous system ; this class includes practically all the skeletal 

 muscles. The involuntary muscle fibers are more or less 



