WITH THE MICROSCOPE. 135 



between the object glass and the thin glass covering the web is very 

 considerable, there is not the same danger of serious derangement 

 every time the animal moves slightly. Several different lengths of 

 tube may be adapted to the microscope body, which may be thus 

 increased to the length of two feet or more, if desired. 



If a small artery be brought into focus and the tip of one of the 

 toes be very lightly touched, the artery is seen to contract imme- 

 diately, aud somewhat irregularly in different parts of its course. 

 Sometimes a few blood corpuscles are firmly compressed, and for 

 several seconds the vessel remains so strongly contracted that not a 

 corpuscle passes along it. By performing this instructive experiment, 

 the observer may realise the effects of the wonderful contractile 

 power of the coats of [the smaller arteries, and demonstrate conclu- 

 sively that the afferent nerve fibres distributed to the skin of the foot 

 generally, influence the nerve centres from which the nerves ramifying 

 amongst the muscular fibres of the arterial coats take their use. This 

 is a beautiful instance of reflex nervous action affecting the vessels. 



The circulation may also be studied during life in the capillaries 

 of the tail of a small fish, minnow, stickleback, eel, carp, &c. The 

 fish should be wrapped up in wet lint and loosely tied at one end of a 

 glass slide, the tail being placed about the centre, and covered with 

 a piece of very thin glass. 



235. of the Action of the Heart. A more correct idea of the 

 mode of action of the heart may be formed by watching its contrac- 

 tions in a small living animal under the microscope than in any other 

 way with which I am acquainted. A young fish, or newt, or frog 

 tadpole may be taken for the purpose, but I have found that a young 

 snake removed from the egg exhibits the phenomena most beautifully. 

 The blood may be distinctly seen as it eddies through the various 

 apertures in passing to or from the different vessels and cavities of 

 the heart. The undulating contractions of the auricles and ventricle 

 of the heart are very wonderful. Under a two-inch power adapted 

 to a binocular microscope, the movements of the heart may be 

 studied most advantageously. 



The circulation in the tadpole has been well described by 

 Mr. Whitney (Trans. Mic. Soc., vol. X, p. i, 1862.) The animal 

 should be starved for a few days before being submitted to examina- 

 tion, in order that the intestine may become transparent. 



The branchice. of the frog tadpole or young newt may be examined 

 in a flat glass eel! specially prepared for the purpose, and by an 

 arrangement of tubes the animal may be supplied with fresh water 

 while it remains under observation. In pi. XVIII, fig. 106, is repre- 

 sented a form of cell which I made some years ago for a proteus, but 



