196 OUTLINES OF EQUINE ANATOMY. 



the phrenic nerve, whicli arises in the neck from branches 

 of the third, fourth, and fifth cervical nerves, commnni- 

 cating with the axillary plexus, and runs backwards over 

 the base of the heart to the diaphragm, that of the left side 

 passing through the posterior mediastinum, the right 

 phrenic nerve accompanying the posterior vena cava in its 

 special fold of pleura. 



We can now, by division of the above-described vessels, 

 remove the heart from the thorax. We find that it is a 

 hollow muscular organ, cone-shaped, with its base upwards, 

 having its apex directed downwards and backwards. It is 

 covered by the visceral layer of the serous pericardium, 

 which gives it a glistening appearance, and through which 

 may be seen the furrows on the surface which mark its 

 division into several cavities. Of these we see one running 

 downwards on each side, ventricular furrow, one^running 

 horizontally around the base of the heart, auriculo-ven- 

 tricular furrow. These contain the vessels of the organ 

 with nerves embedded in a quantity of fat. The two 

 ventricular furrows do not reach the apex of the organ, 

 but meet on the right side a little above that point. 



The heart is divided into two sides ; 7igM or imlmonary 

 heart, left or systemic heart. In the foetus these sides com- 

 municate by the foramen ovale, which pierces the auricular 

 septum, being partially closed by a valvular arrangement 

 of the lining membrane of the heart ; but in the adult, 

 under normal conditions, this becomes impervious, being 

 still recognisable on the right side of the septum as a light 

 circular spot, fossa ovalis, surrounded by a prominent elastic 

 ring, annulus ovalis. Thus, after birth in mammalia, the 

 two sides are distinct ; each side consists of two cavities — 

 an auricle superiorly, a ventricle inferiorly. The heart is 

 lined by serous membrane, endocardium, which is con- 

 tinuous with the serous membrane lining the large vessels, 

 which run to or from its cavities. On the left side, espe- 

 cially in the auricle, we find a layer of yellow elastic tissue 

 between the muscular structure and endocardium. The 

 hasement structure of the heart is situated at the point of 

 junction of the four cavities. Here we find a firm, irregular- 

 shaped piece of fibro-cartilaginous substance, which in the 

 ox is always, in the horse occasionally, ossified ; from this 

 run three yellow elastic rings ; one to the right side, cir- 

 cumscribing the right auriculo -ventricular opening ; another 



