612 GLOSSARY. THOKAX URETHEA. 



THORAX. See Abdomen. 



TICKS. See Parasite. 



TINEA. See Parasite. 



TISSUES. The more or less compound solids out of which the organs 



and parts of the living body are constructed. 

 TRACHEA. The windpipe. 

 TRICHODECTES. See Parasite. 

 TRICUSPID VALVE. The valve commanding the auriculo-ventricular 



orifice on the right side of the heart. 

 TYMPANUM, MEMBRANA TYMPANI, TYMPANIC CAVITY. The drum of 



the ear in which the bones of the ear are lodged. Membrana tym- 



pani, the membrane which divides the drum of the ear from the 



external passage of sound. 



UNITS OF HEAT. A mode of speaking devised to facilitate the ex- 

 pression of the amount of heat developed in the combustion of 

 given quantities of combustible bodies in oxygen, chlorine, and 

 other supporters of combustion. A unit of heat, though at first 

 sight it seems not to be an absolutely definite quantity, yet is 

 really definite, in which it differs from a degree of heat ; for the 

 amount of heat that constitutes a degree depends not merely on 

 the scale of the thermometer used, but in a very great degree on 

 the amount of mercury or spirit contained in the bulb and stem. 

 A unit of heat is the quantity that raises an ounce of water one 

 degree Fahrenheit, or a gramme of water one degree centigrade. 

 Its quantity then, like that of a degree, is not positive but relative 

 to the amount of water which it can raise one degree on the scale 

 employed. An ounce of carbon, by being burnt in oxygen, pro- 

 duces 14,200 units of heat that is, as much heat as can raise 

 14,200 ounces of water through one degree Fahrenheit. It is 

 plain, then, that in this statement a unit of heat signifies the quan- 

 tity required to raise one ounce of water one degree Fahrenheit. 

 Again, a gramme of carbon, by being burnt in oxygen, produces 

 8080 units of heat that is, as much heat as can raise 8080 

 grammes of water through one degree centigrade. 



But it is manifest that a unit of heat is a definite quantity, be- 

 cause the amount of it is the heat required to raise a definite 

 quantity of water through a fixed range of temperature. The 

 modes of expression differ, but the quantities are unalterable. 



UREA. The principal organic constituent of the urine. See p. 310. 



URETER. The name of the canal by which the urine is conveyed 

 from each kidney to the posterior part of the bladder. 



URETHRA. The canal by which the urine issues from the bladder, 

 passing in the male through the yard or penis, and in the female 

 ending at the lower part of the orifice of the vagina or passage 

 leading from without to the uterus or womb. 



