GLOSSARY. 729 



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 



4C. 



Spherical aberration. The different degrees of refraction at differ- 

 ent parts of a lens giving rise to different focal lengths, and causing 



an indistinctness of the image. 



Sphygmograpb. An instrument for obtaining a graphic representa- 

 tion 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 the perspiration. 

 Summation. The adding together 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 building up or construction of a chemical 



compound from simpler materials. Natural processes are not termed 



syntheses. 

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



Taurocholic acid. An acid existing in combination with soda in the 

 bile of man and of carnivorous animals. 



Tetanus. In physiology is used to denote the prolonged contraction of 

 the skeletal muscles which follows rapidly repeated stimulations or 

 nervous impulse. 



Thalamencephalon. The part of the anterior cerebral vesicle which 

 is left after the differentiation of the optic lobes and cerebral hemi- 

 spheres. 



Thrombosis. The occlusion of a vessel by a local coagulation of the 

 blood. 



