CHAP. VI HEART 79 



yellowish fluid appear on its surface and gradually run to- 

 gether 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, tut 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 a vein 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, w), 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. au), but appearing single in 

 the entire heart. Passing obliquely across the auricles is a 

 cylindrical structure, the comis arteriosus {c. art) ; it starts 

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

 forwards and to the left, finally dividing near the anterior 

 boundary of the auricles into two branches, Avhich extend 

 respectively right and left. 



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

 3 and 4), there is seen in the dorsal part of the pericardial 

 cavity a thin-walled chamber (s. v) of a dark colour, con- 



