56 PEACTICAL PHYSIOLOGY 



different rhythm. The sinus is the more injured, and beats at a slower 

 rate. If the cut be made through the auricles, the sinus beat continues 

 and is unaffected by the injury. Cut off the ventricle just above the 

 auriculo-ventricular groove. After a period of inhibition both auricles 

 and ventricle beat. The auricles recover first. Cut through the ventricle 

 below the auriculo-ventricular groove. The apex preparation does not 

 beat spontaneously. It responds to a prick by a beat, and may in some 

 cases be taught to beat rhythmically by rhythmic stimulation. A 

 crystal of common salt placed on the apex or the passage of the 

 galvanic current through the apex preparation provokes its rhythmic 

 contraction. 



Khythmic Contraction the function of the Heart Muscle. Cut 

 out small pieces of the bulbus arteriosus, and place them under the 

 microscope in a watch glass containing Ringer's fluid. The pieces will 

 beat rhythmically. There are few if any nerve cells in the bulbus, 

 and there are certainly none in some of these pieces, so the rhythm 

 must be the function of the heart muscle. 



A frog's heart painted with nicotine (1 per cent, solution) continues 

 to beat. Nicotine paralyses nerve cells (Chapter XVII.). 



Isolated portions of the mammalian heart will beat rhythmically 

 for hours if fed through their nutrient arteries (with oxygenated 

 serum). The serum is supplied from a reservoir which need be raised 

 only slightly above the level of the heart. The preparation and the 

 reservoir of serum are together enclosed in a strong glass chamber. 

 The chamber is kept at body temperature, and into it oxygen is 

 forced up to a pressure of two atmospheres. Under this pressure 

 the oxygen dissolved in the serum reaches a tension which is 

 sufficient to maintain the vitality of mammalian tissues (Porter). 



Demonstration. A hen's egg incubated 24-36 hours is broken, 

 and the contents floated out into a dish. With the aid of scissors 

 and forceps the investing albumin is removed. The beat of the heart 

 may be observed in the embryo under the microscope. The embryonic 

 heart beats before its muscular tissue become differentiated, and before 

 any nerve-cells become included within its structure. The inherent 

 power of rhythmic contraction is seen in the chick heart by the 

 24th hour of incubation, while the migration of ganglion cells into the 

 heart does not take place till the 3rd day. 



The structural elements of the heart are nucleated, branched, and 

 cross-striated cells. The muscle-cells are joined together into net- 

 works and bands, so as to form one functional whole, and hence 

 excitation of any one part leads to the contraction of the whole. The 

 first part to begin to functionate in the embryo is the venous end. In 



