GLOSSARY. SARCOLEMMA SENSIBILITY. 607 



perhaps they nourish the blood-plasma by their decay ; certainly 

 they grow at the cost of the plasma. 



SARCOLEMMA. The structureless membrane enclosing the primitive 

 fibrils of a muscle. 



SCAB. See Parasite. 



SCARF-SKIN. See Derma. 



SEBACEOUS GLANDS. Glands of the skin, composed of an aggregate 

 of small vesicles filled with opaque white substance like soft oint- 

 ment. 



SECRETION. A word used in several different significations. In its 

 largest sense it denotes the separation of any material from the 

 blood, for whatever purpose or by whatever apparatus. When 

 contrasted with excretion, it signifies the separation of fluids of 

 some definite use within the body, while excretion signifies the 

 separation of something that would prove hurtful if retained. In 

 this contrasted sense the pancreatic juice is a secretion, the urine 

 an excretion. If material be ever separated from the blood directly 

 by blood-vessels, in which sense exhalation was formerly used, 

 then secretion, contrasted with exhalation, denotes separation from 

 the blood by glands or a glandular apparatus. 



SEMILUNAR VALVES. The valves in the orifice of the aorta and pul- 

 monary artery by which the blood is prevented from returning 

 into the ventricle during its diastole. 



SENSATION. The state of consciousness that succeeds impressions 

 made on the peripheral extremities of certain nerves. 



SENSIBILITY. The property of transmitting impressions to the nerv- 

 ous centre, so that a sensation may be excited. The sensibility of 

 a tissue depends on the presence of filaments of nerves of sense 

 in that tissue. In short, when a texture or part is said to possess 

 sensibility, it is the local seat of such sensations as belong to the 

 nerves, the filaments of which are therein diffused. It is in the 

 sensibility of such parts or textures that matter and spirit come 

 into union and communication. Thus, though sensations are states 

 of consciousness that is, conditions of spirit yet they have a local 

 seat in every instance. The sensibility of the membrane of the 

 nose to odours depends on the presence of filaments of the olfac- 

 tory nerves spread therein. That the brain is the fountain of the 

 sensation of smell, like that of all other sensations, is indeed an in- 

 ference of physiology, but that the membrane of the nose is the seat 

 of the sensation of smell, in the ordinary apprehension of every 

 individual, is a primitive fact, independent of all speculation. 

 And the same thing is true of all other sensations. When the 

 point of a needle touches a spot on the surface of the skin, the 

 impression the needle makes is felt in that spot ; and yet physiology 

 makes it certain that there is another spot in the brain or nervous 

 centre, by communication with which alone that spot of the skin 



