730 GLOSSARY. 



Infusoria. A name given to a large class of simple organisms which 



are found in dirty water. 

 Irradiation. The phenomenon that bright objects appear larger 



than they really are. It is due to the extension of the effect of 



light on the retina to those parts immediately adjacent to where 



the light rays impinge. 



Kymograph, An instrument used for recording graphically the un- 

 dulations of blood-pressure, measured directly from a bloodves- 

 sel by means of a manometer. 



Lachrymal. Pertaining to the secretion of tears. 



Lacunae. Small spaces in the substance of bone tissue, occupied 

 during life by the bone-cells. They appear black in sections of 

 dry bone owing to their containing air, which replaces the 

 shrivelled cells. 



Latency, or Latent period. The time that elapses between the 

 moment of stimulation and the response given by an active tissue. 



Leucin. This is a common product of the decomposition of proteids. 

 It is formed in the later stages of pancreatic digestion. 



Leucocytes. A term applied to the white blood-corpuscles and lymph 

 cells. 



Lumen. The open space seen on section of a tube, vessel or glandu- 

 lar saccule ; the cavity surrounded by the gland-cells, in which 

 the secretion collects. 



Lymph. The liquid collected by the absorbent vessels from the tis- 

 sues ; the return flow of the irrigation stream escaping from the 

 bloodvessels to nourish the tissues. 



Manometer. An instrument for measuring pressure ; made of a U- 

 shaped tube containing liquid, commonly mercury. 



Medullary sheath. A soft clear sheath around the axis cylinder of 

 medullated nerves, which, owing to its refracting power, gives 

 them the white appearance. 



Menstruation. The monthly change in the mucous membrane of 

 the uterus, which accompanies the discharge of the ovum. 



Meroblastic. The form of ova in which the yolk does not undergo 

 complete segmentation, as that of birds. 



Mesoblast. The middle of the three layers of the blastoderm from 

 which the connective tissues and vascular apparatus of the em- 

 bryo are formed. 



