NERVES OF THE HEART. CARDIAC MUSCLE. 255 



centre from which efferent impulses are in turn sent to the 

 muscular coat of the cutaneous arteries. The blood driven 

 from the skin must find a place elsewhere in the circulatory 

 system, and so internal organs tend to become over-full and 

 at the same time general arterial pressure is raised. This, 

 again through nerves, acts upon the heart, and alters its rate 

 of beat for a time. But in health some internal arteries soon 

 dilate sufficiently to compensate for the constriction of the 

 surface vessels, and arterial pressure and the pulse again be- 

 come normal, though with a less proportion of the total blood 

 flowing through the skin than before: this readjustment is 

 brought about entirely through nerves and nerve-centres 

 placing all the arteries in connection with one another and 

 with the heart, so that they exert a mutual control. If the 

 cold be not too prolonged its cessation is followed by a return 

 of the blood-flow to its original condition, this action being 

 brought about by cardiac and vascular nerve apparatuses. We 

 have to mainly consider in this and the succeeding chapter 

 the nerves which regulate the heart-beat and those which in- 

 fluence the calibre of arteries; but it is necessary iirst to study 

 the muscular tissue of the heart more thoroughly than we have 

 hitherto done. 



Some Physiological Peculiarities of Cirdiac Muscle. 

 We have already seen that the muscular tissue of the heart, 

 though striped, differs considerably in structure from the 

 tissue of the skeletal muscles: it differs also somewhat in 

 properties, and as the latter differences can be most readily 

 studied on the heart of the frog, which will beat for a long 

 time after excision, it will be best to commence with that. 

 The frog's heart consists of four contractile chambers through 

 which the blood flows successively, as is indicated in the dia- 

 gram, Fig. 99, in which no attempt has been made to indicate 

 the actual appearance of the organ, which is in fact curved 

 on itself somewhat in the form of a capital si (see Z, Fig. 9.9), 

 and this is also the shape of the mammalian heart in an early 

 stage of embryonic development. The main chambers are 

 incompletely separated by constrictions, at some of which 

 valves are placed, and are in order the venous sinus, A, 

 receiving blood from the systemic veins; the atrium, consist- 

 ing of two auricles, B, C, of which the right is much the 

 larger and is supplied from the sinus, while the left gets blood 

 from the small pulmonary veins, pv, the ventricle, D, sup- 



