500 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



mammals into four cavities, two auricles and two ventricles. In its 

 function it acts as a force-pump, and by the contraction of its walls 

 empties the contents of its cavities into the arterial branches in connec- 

 tion with them, and therefore is the starting point of the circulation. Its 

 action as a pump is therefore mainly physical, the effect produced by its 

 contractions being governed by their frequency, force, and character, 

 and the quantity of blood ejected at each beat. Its physical properties, 

 as indicated by the above considerations, are governed by a number 

 of vital conditions, which modify the various characters of cardiac 

 pulsation. These will later demand attention. 



The heart consists of interlacing muscular fibres, which are midway 

 between the striped and nnstriped muscular tissues. They are 

 extremely short, transversely striated, are devoid of sarcolemma, and 

 are usually bifurcated at their extremities and anastomose with each 



FIG. 195. ARRANGEMENT OF MUSCULAR FIBRES IN THE AURICLES AND 

 GREAT VEINS. (Landois.) 



I, muscular fibres on the left auricle, showing the outer transverse and inner longitudinal fibres, and 

 the circular fibres on the pulmonary veins, v.p. V, the left ventricle (Reid). 



II, arrangement of the striped muscular fibres on the superior vena cava (Elischer). a, opening of 

 vena azygos ; v, auricle. 



other. In the tubular heart of the embryo, microscopic inspection 

 shows that the muscular fibres may at first be resolved into an outer 

 circular and an inner longitudinal layer of fibres. And, from the 

 fact that at first the heart consists of but one chamber, it is clear 

 that a part of the fibres at least must be common to the two 

 auricles and a part also to the two ventricles. While, however, this is 

 true, the muscular fibres of the auricles are completely separated from 

 those of the ventricles by the fibro-cartilaginous rings of the auriculo- 

 ventricular orifices. In adult animals the fundamental circular arrange- 

 ment of the embryonic fibres partly remains, while in the ventricles, as 

 the septum becomes formed, the fibres become twisted into the form 

 of spiral loops. The arrangement of the muscular fibres in the differ- 

 ent cavities of the heart serves to a considerable degree to explain the 



