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 whicTi 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 in the 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. 



