Molecules. The smallest physical particles of matter that can exist in a separate state. They are probably always constituted of two or more

atoms.

Morphology. The science which treats of the forms and structures of living beings.

Morula. The stage of development of the ovum after segmentation, in which all the young cells are alike, before the blastoderm is formed. Mucin. The characteristic constituent of mucus.

Müllerian duct. An embryonic structure from which are formed the genital passages in the female, viz., Fallopian tube, uterus and vagina.

Mydriasis. A dilated state of the pupil.

Myograph. An instrument for graphically recording muscle contraction. Myopia. The condition in which the focus of parallel rays of light falls short of the retina; short sight.

Myosin. The substance formed by the coagulation of muscle plasma. It is one of the globulins.

Natural nerve currents. The electrical currents passing through an exposed muscle or nerve while in a state of rest.

Neuroglia. The reticular connective tissue which binds together the elements of the nerve centres.

Non-polarizable electrodes. Electric terminals specially constructed so as not to set up secondary currents on application to moist living tissues. 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 protoplasm, commonly round, but sometimes elongated, as in muscle.

Odontoblasts. 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 sense of smell.

Omphalo-mesenteric. The vessels connecting the embryonic circulation

with the yolk sac, which are early obliterated in the mammalian fœtus. 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. Osteoblast. The active cells in forming bone.

Osteoliths. Calcareous particles lying in the endolymph.

Oxyhæmoglobin. The coloring matter of the blood corpuscles.

Paraglobulin. One of the more abundant albumins of the blood-serum globulia.

Paramecium. A unicellular organism composed of a soft mass of protoplasm enclosed in a firmer case, and covered with motile cilia.

Parapeptone. A body produced in gastric digestion during the formation of peptone.

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 a 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 progres sive constriction, and so force the contents of the tube onward.

Phakoscope. 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; used in physiology to indicate chemically complex kinds of matter which subserve to the forma tion of the living tissues.

Poikilothermic. Varying in temperature. A term applied to those animals whose temperature varies with that of the surrounding medium-" coldblooded animals."

Presbyopia. A loss of power of accommodation for near vision which accompanies old age.

Prosencephalon. That part of the developing anterior cerebral vesicle from which are formed the olfactory and optic lobes, the hemispheres, and corpora striata and optic thalami.

Protista. A large group of organisms which remain in the primitive state of a single cell during their lifetime.

Protococcus. A unicellular vegetable organism, the protoplasm of which contains chlorophyll.

Protoplasm. The substance which gives rise to the primitive vital phenomena, seen in unicellular organisms, and which is the chief agent in executing the functions of all the active tissues.

Protovertebrææ. The primitive segments of the mesoblast in the site of the future vertebral column.

Protozoa. That division of the protista which has been assigned to the animal kingdom.

Proximal. A term used to denote a part relatively nearer to the centre. Pseudopodia. Projections thrown out by moving protoplasm, by means of which cells, such as amoebæ, move.

Ptosis. Drooping of the eyelid accompanying paralysis of the third nerve. Ptyalin. The ferment of the saliva. In a weak alkaline solution it converts starch into dextrine and sugar.

Reflex action. The activity caused by a ganglion cell reflecting an afferent impulse along an efferent nerve to the neighborhood of original stimulation. Reflexion. The return of rays of light from a surface.

Refraction. The bending which rays of light undergo when passing obliquely from one medium to another of different density.

Reticulum. A network; a term applied to the interlacement of fibres, seen in reticulated connective tissue, etc.

Rheoscopic frog. An arrangement by which the change in the electric current of one muscle of a frog is made to act as a stimulus to the nerve of another.

Saponification. The formation of soap; the decomposition of oils or fats by means of alkalies into salts of the fatty acids and glycerine.

Sarcolactic acid. The principal acid in dead muscle. It has a dextro-rotatory power on polarized light, which ordinary lactic acid does not possess. Sarcolemma. The delicate sheath surrounding the fibres of skeletal muscles. Sclerotic. The fibrous coat of the eyeball.

Sensorium. That part of the nerve centres supposed to receive sensory impressions.

Somatopleure. The subdivision of the mesoblast which, with the attached epiblast, forms the body walls of the embryo.

Specific gravity. The relation of the weight of a given volume of any substance to the weight of an equal volume of distilled water at 4° C.

Spherical aberration. An indistinctness of the image caused by the difference in refraction at the centre and margin of a lens giving rise to different focal lengths. Sphygmograph. An instrument for obtaining a graphic representation of the pulse wave by means of a lever applied to the radial artery at the wrist. Splanchnopleure. The subdivision of the mesoblast which, with the attached hypoblast, forms the chief visceral cavities of the embryo.

Sporadic ganglia. Swellings occurring in the course of the peripheral nerves caused by a group of nerve corpuscles.

Steapsin. A ferment existing in the pancreatic juice which causes or aids the saponification of the fats.

Sudoriferous glands. The small tubular glands of the skin which secrete perspiration.

Summation. The fusion of several single contractions of muscle to form a tetanic contraction; the accumulation of stimuli.

Sutures. Unions formed by the direct apposition of bones without intervening cartilage. They do not permit of motion.

Sympathetic nerve. The ganglionic nervous cord on either side of the vertebral column. It transmits most of the vasomotor impulses coming from the cerebro-spinal centres.

Symphysis. A form of joint without synovial membrane in which the bones are fixed together by fibro-cartilage.

Synthesis. The artificial construction of a chemical compound from simpler materials.

Systole. The period of contraction of the heart's muscle.

Taurocholic acid. An acid existing in combination with soda in the bile. Tetanus. In physiology is used to denote the prolonged contraction of the skeletal muscles which follows rapidly repeated stimulations or nervous impulse. Thalamencephalon. That part of the anterior cerebral vesicle which is left after the differentiation of the optic thalami, cerebral hemispheres, etc. Thrombosis. The occlusion of a vessel by a local coagulation of the blood. Trabecula. Supporting bars of tissue passing through some organs, such as those proceeding from the capsule to the interior of the spleen or lymphatic glands.

Trophic. Relating to nutrition.

Trypsin. A ferment in the pancreatic juice which in alkaline solutions converts proteids into peptones.

Tyrosin. A substance formed together with leucin during pancreatic digestion; it is also produced by putrefaction of proteids.

Urachus. The bond of union which at an early period connects the urinary bladder with the allantois in the embryo; it is subsequently obliterated in the foetus.

Vacuoles. Small cavities occurring in cells. They are supposed to have im portant functions in the unicellular organisms.

Vagus. The part of the eighth pair of nerves distributed to the viscera of the throat, thorax and abdomen; the great regulating nerve of the vegetative functions.

Vaso-constrictor. Those impulses which excite contraction of the vascular muscles.

Vaso-dilator. Those impulses which inhibit the action of the vascular muscles.

Vasomotor. Those nervous mechanisms controlling the movements of the blood vessels.

Villus. A hair-like process. A term applied to the small projections characteristic of the small intestine. They contain blood vessels and lacteals, and are important in absorption.

Vitellus. The yolk of the ovum, which in mammals divides completely to form the embryo. In birds only a part divides, and the rest serves to nourish the chick.

Vorticella. Bell animalcule, a bell-shaped unicellular organism with a rudi. mentary, ciliated mouth cavity and rapidly contractile stalk.

Wolffian body. An embryonic structure, the forerunner of certain parts of the genito-urinary apparatus.

Zymogen. A peculiar substance supposed to give rise to the pancreatic ferments.