GLOSSARY. 723 



Ductus venosus. A vessel which, in the foetus, carries blood from 

 the umbilical vein to the vena cava. After birth it becomes a fibrous 

 cord. 



Ductus vitello-intestinalis. The union between the yelk sack and 

 the intestine of the embryo. 



Dyspnoea. Difficulty of breathing ; it is a condition in which inor- 

 dinate respiratory movements are excited by an unusually venous 

 state of the blood in the respiratory nerve centre. 



Ectoderm. The outer layer of simple organisms. 



Ectosarc. A term applied to certain unicellular organisms, meaning 

 the outer layer or covering. 



Electrodes. The two terminals which are applied to a substance in 

 order to complete the circuit when it is required to pass a current 

 through the substance. 



Electrotonus. A peculiar electric state of nerves resulting from the 

 passage of a continuous current through them. 



Embryo. The name given to the animal at the earliest period of its 

 development. 



Emmetropic. A term applied to the normal eye, in which parallel 

 rays of light are brought to a focus at the retina without accommo- 

 dation. 



Emulsiflcation. The suspension of very fine particles of liquid or 

 solid in a liquid which is not able to dissolve them. 



Endoderm. The inner layer of simple organisms. 



Endogenous reproduction. The formation of new cells or organ- 

 isms within the body of the parent individual. 



Endolymph. The liquid contained within the membranous labyrinth 

 of the ear. 



Endosmosis. The diffusion of a fluid into a vessel through its walls 

 from the exterior. 



Endothelium. The single layer of thin cells which lines the serous 

 cavities, the lymphatic and blood vessels, and all spaces in the con- 

 nective tissues (mesoblastic lining cells). 



Epiblast. The uppermost of the three layers of the blastoderm from 

 which the epidermis and the nerve centres are developed. 



Epithelium. The non- vascular cellular tissue developed from the epi- 

 and hypoblast of the blastoderm. 



Eupncea. A term used to denote the normal rhythm of respiratory 

 movements in contradistinction from dyspnoea and apncea. 



Excito-motor. Impulses which, reflexly, call forth motion. 



Excito-secretory. Impulses calling forth the activity of gland cells, 

 commonly applied to afferent influences which act reflexly. 



