44 THE FROG 
The centrale is represented by a small bone—a mere nodule—on 
the preaxial side of the distal end of the tibiale; it supports the 
prehallux (see below). Between the two proximal tarsal bones 
and the metatarsals are two very small irregular tarsal bones 
corresponding to the distal row of carpal bones in the wrist. Of 
what advantage to the frog is the elongation of the two proximal 
tarsal bones? 
The foot or pes exclusive of the tarsus has five complete 
digits and a small supernumerary digit. The supernumerary 
digit is on the preaxial side of the foot and consists of from one 
to three small bones; it is sometimes called the 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 (XX 4) the foot including the ankle. 
Make a table showing the homologies between the elements of 
the fore-limbs and the hind-limbs. 
XIV. 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 containing many nuclei; (0) 
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 un- 
branched, are lacking in cross-striations, and contain each a single 
nucleus. : 
With regard to nervous control, muscular tissue may be classi- 
fied as voluntary and involuntary. The voluntary muscle fibers 
