GLOSSARY. 727 



Notochord. The primitive vertebral axis of the embryo. 

 Nucleolus. A small spot observable in some nuclei. 

 Nucleus. A central part of a cell differentiated from the main proto- 

 plasm, commonly round, but sometimes elongated as in muscle. 



Odontoblast. Living cells lining the pulp cavity of the interior of a 

 tooth, and presiding over the growth and nutrition of the dentine. 



Olfactory. Pertaining to the special sense of smell. 



Omphalo-mesenteric. The vessels connecting the embryonic circu- 

 lation with the yelk sack, which are early obliterated in the mamma- 

 lian foetus. 



Ophthalmoscope. An instrument consisting of a small mirror by 

 which the interior of the eye can be illuminated so that the fundus 

 may be viewed. 



Optic cup. The involuted optic vesicle which is developed into the 

 retina, etc. 



Oxyhaemoglobin. The coloring matter of the red blood corpuscles. 



Paramo33ium. A unicellular organism composed of a soft mass of 

 protoplasm inclosed in a firmer case and covered with motile cilia. 



Parapaptone. A stage in the formation of peptone produced in gas- 

 tric digestion. 



Pepsin. A ferment existing in the gastric juice which converts proteids 

 into peptones. 



Peptone. A form of albumin which is produced during the digestion 

 of proteids ; it is very soluble, and diffuses readily through membrane. 



Perilymph. The liquid surrounding the membranous labyrinth of the 

 ear. 



Peristalsis. The mode of contraction of the muscular walls of certain 

 tubes, as the oesophagus and intestine, the effect of which is to 

 cause a progressive constriction, and so force the contents of the tube 

 onward. 



Phakosoope. An instrument for estimating the changes in the shape of 

 the lens during accommodation by doubling the reflected images with 

 a prism. 



Placenta. The intra-uterine organ by means of which the foetal blood 

 is brought into close relationship to that of the mother, so as to gain 

 nutriment and oxygen and get rid of effete matters. 



Plasma. A term meaning anything formed or moulded ; it is applied 

 in physiology to indicate chemically complex kinds of matter which 

 subserve to the formation of the living tissues. 



Poikilothermic. Varying in temperature. A term applied to those 



