390 PRACTICAL ANATOMY. 



tion is to be that which an animal substance will bear without altering 

 its texture, or at least very little ; and the more fluid it is in this degree 

 of heat, joined with a sufficient degree of solidity in a heat of 70° or 

 80°, the greatest heat of the climate of Great Britain, it is so far a 

 proper injection. Another property when cold, besides solidity, is 

 tenacity ; for there are many substances which, when cold, become very 

 brittle, and will not bend or yield to pressure, which renders them very 

 unfit for casts of this kind ; nor should they be such as will alter in 

 tenacity by time, which is an effect arising from most substances that 

 are volatile, therefore are bad ingredients in such injections ; however, 

 in some cases they may be admitted with advantage, as will be men- 

 tioned. Injections for such purposes shoidd be such as do not imme- 

 diately become solid upon being exposed to cold, or at least that degree 

 of heat that the part can bear which is to be injected, for there are 

 many substances that can be made very fluid by heat, but in the degree 

 of 60° become immediately solid. "Wax and tallow are of this kind. 



As it will very often happen that the corroding injection may run 

 more minutely in some vessels than others, it beeomes necessary that 

 we should be able to adapt our injections to tbe various purposes. This 

 will in general arise from the smallness of the vessel, in proportion to 

 its length, which is to be injected. If it is a part that has several 

 vessels, some very large, and others veiy small, as in the liver, the 

 large, viz. the vena portarum and vena cava, should be injected with 

 firm injection, as a support to the small, and tbe small vessels, viz. the 

 artery and duct, may be injected with a softer injection. 



Wax, resin, turpentine varnish, and tallow, in proportions according 

 to the kind of preparation, form the menstruum or body of the injection 

 for receiving, when melted, the required colour. 



The right consistency of our injection may be known by dropping a 

 little of it, when melted, into cold water ; and when soft, form it into 

 the shape of a vessel, and put it into the water again : when quite cold, 

 try to bend it. If it breaks, it is too hard ; if it bends very easily, it is 

 too soft. It must be of such a consistence as not to bend without some 

 force. If it breaks, then you may be certain that there is too much 

 wax ; therefore some resin and perhaps a little tallow should be added. 

 If it be too soft, then some more wax should be added. Those that 

 have the turpentine varnish, the Yenice turp., or oil of turpentine in 

 them, must be tried every time they are heated ; because they lose part 

 of their volatile oil, and become too hard and brittle, and therefore want 

 an addition of one of these ingredients. 



Of the Colours. — To these injections add as much colour as makes 

 them appear of a proper bright colour to the eye. Those generally used 



