388 VETERINARY PHYSIOLOGY 



the ventricles, extend freely upwards to terminate in a series 

 of tendinous cords (the chordae tendineae), which are inserted 

 partly into the auriculo-ventricular valves, presently to be 

 described, and partly into the auriculo-ventricular rings. 

 The papillary muscles are merely specially modified columnae 

 carnea^. In many cases, actual muscular processes extend 

 from the apex of the papillary muscles to the auriculo- 

 ventricular ring. 



In the left ventricle there are two papillary muscles, or 

 groups of papillary muscles, one in connection with the 

 anterior wall of the ventricle, and one in connection with 

 the posterior Avail. 



In the right ventricle there are — (1) One or more small 

 horizontally running papillary muscles just under the pul- 

 monary orifice, their apices pointing backwards — (fig. 165, 

 S.P.M.). (2) A large papillary muscle taking origin from 

 the mass of fleshy columns at the apex of the ventricle 

 (A. P.M.). (3) One or more papillary muscles of varying 

 size arising from the posterior part of the apical portion of 

 the ventricle (P. P.M.). (4) A number of small septal 

 papillary muscles arising from the septum. 



The distribution of the chordse from these muscles will be 

 considered in connection with the auriculo-ventricular valves. 



In contraction, the outmost and inmost layers of the 

 ventricles tend to approximate the apex to the base of the 

 ventricles, but this is resisted by the contracting middle 

 layer. The apex tends to be tilted towards the right, the 

 papillary muscles shorten, the columnse carnese by their 

 shortening and thickening encroach upon the ventricular 

 cavity, and help to abolish it, while the auriculo-ventricular 

 rings are drawn inwards towards the septum and downwards. 



2. The Distribution of Neurons in the Heart.— (1) In the 

 frog's heart the nervous structures are generally described as 

 distributed in three groups or ganglia, but the separation 

 of these from one another is artificial (fig. 166). 



(a) In the wall of the sinus venosus a plexus of nerve 

 cells and nerve fibres constitutes the ganglion of the sinus 

 (Remak's ganglion). 



