April 19, 1918J 



SCIENCE 



397 



upon the specimen while it is being kept in 

 the refrigerator should be changed at least 

 once a day. 



However, the percentage of acid below 6 

 per cent, may be varied considerably. In- 

 deed, 3 parts of acid to 1,300 parts of water 

 has been found to bring to distinct view 

 cutaneous and other nerves lying near the 

 surface, if the animal is left in the solution 

 three op four days. Interior parts, as the 

 muscles of the eye, then appear reddish, but 

 may be made almost transparent by addition 

 of dilute acid directly to the dissected part as 

 the head lies covered with cold water in the 

 dissecting pan. 



Doubtless several factors are involved, in 

 deciding strength of acid to be used. Chief 

 of these is the size of the animal, the nature 

 of the tissue and the location of parts to be 

 studied. 



The use of this acid method for investigating 

 the anatomy of various animals, together with 

 the best means of preserving them over long 

 periods, is under investigation and will be 

 treated in a future paper. 



In the 5 per cent solution muscles are not 

 only separated from each other but the fiber 

 bundles, of which they are composed, are 

 brought out distinctly, by the breaking down 

 of connective tissue between them. The en- 

 tire muscle thus made more or less transparent, 

 allows its smaller nerves to appear. 



An excellent illustration of the advantage 

 of this transparency was found in the case of 

 the orbicularis oculi. After treatment in the 

 acid solution, following the removal of the 

 skin peripheral to the eyelids, little further 

 dissection was needed for study. The orbital 

 and palpebral portions of the muscle, their 

 constituent fibers, with the ramifications and 

 anastomoses of motor and sensory nerves 

 within them, were distinctly observable 

 throughout the breadth and depth of the 

 muscle. 



The effect on the muscles of the body wall 

 is such as so to separate the constituent parts 

 and so increase their transparency that the 

 smaller divisions of the nerves within the 

 muscle can be observed at various depths. 



Action of the acid seems to continue after 

 removal of the specimen from the solution. 

 In the preliminary treatment, the blood vessels 

 are easily followed and veins can be distin- 

 guished from arteries, while anastomoses of 

 blood vessels upon the walls of the alimentary 

 tract, for instance, are brought out clearly. 

 But, as the action of the fluid (5 per cent. 

 nCl) continues for some days, red corpus- 

 cles are gradually dissolved and the smaller 

 vessels become less clearly discernible. After 

 a week or so, the inner lining of the stomach 

 wall is loosened from the outer layers of 

 muscle and the latter is broken up into its 

 longitudinal, circular and oblique fibers. 

 Later stages of treatment show clearly the 

 interlacing of the muscle fibers of the heart. 



The skeletal parts are found, on removal 

 from 5 per cent, acid, to be decalcified suffi- 

 ciently to yield readily to cutting with scis- 

 sors or breaking with forceps, so that the 

 dissection of nerves where they pass through 

 bony channels is rendered easy. On the 

 whole the tissues are broken down so very 

 gradually that a single specimen can be used 

 for weeks. 



The most fortunate feature of the method 

 lies in the fact that connective tissue is the 

 first to be seriously attacked by the acid. 

 Much of it remains even to the later stages 

 of dissolution, but appears less dense while 

 its strands become so weak that it is readily 

 separable from parts which it holds together. 

 Nerves and muscles, on the other hand, are 

 about the last of the soft parts to be broken 

 down. 



Small unmedullated sympathetic fibers, 

 however, are not favorably affected for dis- 

 section by this method and consequently are 

 not as easily traceable as are medullated fibers. 

 That sympathetic fibers are not dissolved by 

 the solution is certain, since the larger ones, 

 and even a few of the smaller ones related to 

 the blood vessels in the orbit, can be traced 

 with accuracy for some distance. This method, 

 therefore, cannot be recommended for study 

 of the sympathetic system, other than of its 

 grosser parts. In such investigations it is 

 decidedly useful, in locating all the larger 



