G. GENERAL NATURAL HISTORY AND ZOOLOGY. 315 



the extremity below any particular articulation can be made 

 to swing within its normal limits of flexion, supported by 

 the pressure of the air alone. Thus, as Professor Aeby ex- 

 presses it, " When it is found that the arm will hang com- 

 pletely disarticulated in the shoulder-joint, the fore-arm in 

 the elbow-joint, or the hand and fingers in their respective 

 joints, no further proof is required that the ordinary teach- 

 ing with regard to the relation of air-pressure to the joints 

 is entirely erroneous." Dr. F. Schmid has also lately found 

 by experiment that the atmospheric pressure which retains 

 the surfaces of the hip-joint in contact is not only sufficient 

 to support the lower extremity, unaided by muscles or liga- 

 ments, but even to carry an additional burden equal to a 

 third part of the weight of the leg. 20 A, May 15, 1875, 529. 



THE PHYSIOLOGICAL ACTION OF LIGHT. 



In a memoir on the physiological action of light, by Pro- 

 fessors Dewar and M'Kendrick, these authors have especially 

 directed their attention to the effect produced on the retina 

 and optic nerve. Their inquiry divided itself into two parts : 

 first to ascertain the electro-motive force of the retina and 

 the nerve; and, second, to observe whether this were altered 

 in amount by the action of light. They conclude that the 

 action of light is such as to alter the amount of the electro- 

 motive force to the extent of from three to seven per cent, 

 of the total amount, so that a strong flash of light, lasting 

 the fraction of a second, produces a marked effect. A lamp 

 held at a distance of four or five feet, and, equally, the light 

 of a small gas flame, after passing through a depth of twelve 

 inches of a solution of salts of copper and potash, also pro- 

 duce sensible effects. When a diffuse light is allowed to fall 

 on the eye of a frog, after the latter has arrived at a tolera- 

 bly stable condition, the natural electro-motive power is in 

 the first place increased, then diminished. The effects in 

 question are caused by those rays of light that appear to be 

 the least luminous namely, the yellow and the green. 1 A, 

 XXIX., 258. 



ACID OF THE GASTRIC JUICE. 



Dr. Roberteau has lately ascertained that the acidity of 

 the gastric juice is due to hydrochloric acid, and not to lactic 



