CHAP. VI 



HEART 79 



begins to shrink, drops of yellowish fluid appear on its 

 surface and gradually run together into larger and larger 

 drops. The jelly contracts still further, and finally 

 draws itself away from the walls of the vessel and floats 

 in the accumulated fluid, still retaining the form of the 

 vessel, but being greatly reduced in size. The process 

 of coagulation of the blood is now complete ; the red, 

 jelly-like substance is called the clot, the yellowish 

 fluid the serum. 



When first drawn from most veins the blood is deep 

 purple in colour, and the clot retains for a time the same 

 hue. But before long all parts of it which are fully 

 exposed to the air take on a bright scarlet colour. We 

 may therefore distinguish between red, or aerated, and 

 purple, or non-aerated blood. 



Lymph also coagulates on standing, producing a 

 colourless clot. It is practically blood minus its peculiar 

 red colouring matter, the properties of which, as well 

 as the real nature of coagulation, will be discussed in 

 the next chapter. 



The Heart : external characters. Some of the divisions 

 of the heart have already been noticed (p. 20). The 

 ventricle (Figs. 3, 4, 7, 20, 21, and 22, v, vt) is a conical 

 body of a pinkish colour, having its bluntly-pointed 

 apex directed backwards. To its broad base is attached 

 the dark-coloured, thin-walled auricular division, actually 

 consisting of two chambers, the right and left auricles 

 (r. au, I. an], but appearing single in the entire heart. 

 Passing obliquely across the auricle is a cylindrical 

 structure, the conns arteriosus (c. art) ; it starts from 

 the right side of the base of the ventricle, and passes 

 forwards -*hd to the left, finally dividing near the anterior 

 boundary of the auricles into two branches, which 

 extend respectively right and left. 



By lifting up the ventricle, or turning it to one side 



