AM) lil.nnl) I'RKSSrKK 



117 



the artery quickly becomes rigid and contracted, and circular in 

 section, with a calibre greatly lessened, the diameter being re- 

 duced to 2 or 3 mm. or less. The artery becomes often so stiff that 

 a piece 7 or 8 mm. long may be held by one end in an almost 

 horizontal position. Arteries left undisturbed in situ for a day or 

 two after death and then exposed may show little sign of contrac- 



B2 



Qchichs 



W3 



llc/liC/IS 



Fio. 7. Ba, white ovary grafted on black ; fertilisation by white cock. 

 ovary grafted on white ; fertilisation by black cock (Guthrie). 



black 



tion at first, but under the influence of manipulation may soon 

 contract strongly and persistently, the contracted state lasting for 

 days. John Hunter noticed that the arteries of the umbilical cord 

 contracted up to the third day, and not on the fourth. Freezing 

 relaxes permanently the arteries, so that if the " pluck " of a 

 freshly killed ox be bought, and one piece of carotid be taken and 

 frozen on a freezing microtome, and another be manipulated, a 

 striking contrast between the two becomes demonstrable. 



