62 ANATOMY OF THE RABBIT. 
organ for running, though not so highly specialized as that of the 
horse. The human hand retains a fairly primitive form as to its 
general proportions, but is modified into a seizing or grasping type, 
the thumb being opposable to the remaining digits. All three kinds 
of limbs are, however, in themselves, modifications of a primitive, 
five-toed limb, sometimes termed the ideal pentadactyl planti- 
grade type, in which the palm of the hand or sole of the foot is 
placed flat on the ground. The composition of this primitive limb, 
traceable in one form or another throughout the higher vertebrate ; 
and also the serial homologies of the parts are indicated in Fig. 32. 
THE MUSCULAR SYSTEM. 
The contractile tissues are not arranged in a definite continuous 
system as is the case with most other organ complexes of the body. 
Smooth or involuntary muscle fibres, modified mesenchyme cells 
of the embryo, which are under the control of the sympathetic 
INVOLUNTARY nervous system, form the muscle coats of the 
MUSCLE. digestive tube, and are important not only for 
its repeated, peristaltic movements, but also for 
its elasticity and expansive power. Smooth muscle is also a con- 
stituent of many other visceral organs, especially glands, in the 
active secretion of which it appears to play a mechanical part. It is 
further distributed through the walls of the bloodvessell, especially 
the arteries, where it forms the mechanical organ of the vasomotor 
function. This consists inthe control of the diameter of the vessels 
by vasodilator and vasoconstrictor nerves connected with the 
vague nerve and the sympathetic nervous system. The constrictive 
action is stimulated by secretion of the suprarenal glands. Vessel 
regulation of this kind is important, first, in maintaining tone and 
therefore pressure, and, second, in controlling heat loss from the 
surface of the body. Action of the vasomotor nerves may be demon- 
strated physiologically in a variety of ways. Transection of the 
cervical sympathetic nerve of one side in the living rabbit is 
followed by vasodilation of the ear, the congestion of which can be 
seen, and the heat loss is demonstrable by feeling with the hand. 
Stimulation of the cephalic cut end is followed by vasoconstriction. 
