FEATURES OF VASCULAR APPARATUS. 133 



heart are rhythmically contracting, the branches of the pneumogastric nerve 

 going to the heart be adequately stimulated, for instance with the interrupted 

 current, the heart will stop beating ; and that not because the muscles of 

 the heart are thrown into a continued tetanus, the rhythmic alternation of 

 contraction and relaxation being replaced by sustained contraction, but be- 

 cause contraction disappears altogether, all the muscular fibres of the heart 

 remaining for a considerable time in complete relaxation and the whole heart 

 being quite flaccid. If a weaker stimulus be employed the beat may not be 

 actually stopped, but slowed or weakened. And, as we shall see, there are 

 many other cases where the stimulation of efferent fibres hinders, weakens, 

 or altogether stops a movement already in progress. Such an effect is called 

 an inhibition, and the fibres stimulation of which produces the effect are 

 called " inhibitory " fibres. 



The phenomena of inhibition are not, however, confined to such cases as 

 the heart, where the efferent nerves are connected with muscular tissues. 

 Thus the activity of a secreting gland may be inhibited, as for instance when 

 emotion stops the secretion of saliva, and the mouth becomes dry from fear. 

 In this instance, however, it is probable that inhibition is brought about not 

 by inhibitory impulses passing to the gland and arresting secretion in the 

 gland itself, but rather by an arrest, in the central nervous system, of the 

 nervous impulses which, normally, passing down to the gland, excite it 

 to action. And, indeed, as we shall see later on, there are many illus- 

 trations of the fact that afferent impulses reaching a nervous centre, 

 instead of stimulating it to activity, may stop or inhibit an activity previ- 

 ously going on. In fact it is probable, though not actually proved in every 

 case, that wherever in any tissue, energy is being set free, nervous impulses 

 brought to bear on the tissue may affect the rate or amount of the energy 

 set free in two different ways ; on the one hand, they may increase or quicken 

 the setting free of energy, and on the other hand, they may slacken, hinder, 

 or inhibit the setting free of energy. And in at all events a large number 

 of cases it is possible to produce the one effect by means of one set of nerve 

 fibres and the other effect by another set of nerve fibres. We shall have 

 occasion, however, to study the several instances of this double action in the 

 appropriate places. 



CHAPTER IV. 



THE VASCULAR MECHANISM. 

 THE STRUCTURE AND MAIN FEATURES OF THE VASCULAR APPARATUS. 



100. THE blood is the internal medium on which the tissues live ; 

 from it the tissues draw their food and oxygen, to it they give up the pro- 

 ducts of waste matters which they form. The tissues, with some few excep- 

 tions, are traversed by, and thus the elements of the tissues surrounded by, 

 networks of minute thin-walled tubes, the capillary bloodvessels. The ele- 

 mentary striated muscle fibre, for instance, is surrounded by capillaries, 

 running in the connective tissue outside but close to the sarcolemma, arranged 

 in a network with more or less rectangular meshes. These capillaries are 

 closed tubes with continuous walls, and the blood, which, as we shall see, is 

 continually streaming through them, is as a whole confined to their channels 

 and does not escape from them. The elements of the tissues lie outside the 

 capillaries and form extra-vascular islets, of different form and size in the 



