258 HISTOLOGY 



The nerve elements demand the skillful use of silver and methylene blue. 

 The combined maceration and section method outlined in the last chap- 

 ter is useful in studying the retina and visual epithelia of many eyes. 

 For the more general histological relations the ordinary paraffin sections 

 will do when the eye is one of the simple ones whose tissues are homo- 

 geneous. In perhaps the majority of cases, however, the lens acts as a 

 formidable obstacle to the securing of perfect or even fair sections. The 

 lens can seldom be softened, and the best way is to remove it by careful 

 dissection after the tissue is fixed and before the embedding has been 

 begun. With the lens removed very good general sections can be cut, 

 especially in celloidin, in which case, however, the sections are apt to be 

 too thick. In the arthropod eye the difficulty is not the lens but the 

 cuticle, and this is not so formidable an obstacle. The cuticle can some- 

 times be removed, especially when it is very thick. At other times it 

 can be cut in the case of a thin-shelled or newly moulted animal. For 

 general studies of the tissues Zenker's fluid or chrom-aceto-formol is 

 the best. For a good picture of the retina Flemming's strong fixative 

 or some other fluid containing osmic acid is the best. 



LITERATURE 



ENTZ, G. " tiber Inf usiorien des Golfe von Neapel," Mil. d. Zool. St. zu Neapel, Band V, 



1884. 



SHARP, B. "The Eyes of Lamellabranchiata," Mi. zu Neapel, 1886. 

 SCHEWIAKOFF, WALD. " Beitrage zur Kentniss des Acalephanges," Morph. Jahrb., Band 



XV, pp. 21-60, 1889. 



PFEFFER, W. "Die Sehorgane der Seesterne," Zool. Jahrb., Band XIV, 1901. 

 HESSE, RICH. Articles on the "Eyes of Invertebrates" in Zeits. f. Wiss. Zool., several 



recent volumes. 



WATASE, S. "The Eye of Limulus," Studies from the Biol. Lab., Johns Hopkins Uni- 

 versity, Vol. IV, No. 6. 

 SARASIN, P. B. and C. F. " Uber einen mit Zusammengesetzten Augen gedeckten Seeigel," 



Zool. Am., Band VIII, Nr. an, 1885. 

 PHILLIPS, E. F. "Structure and Development of the Compound Eye of the Bee," Proc. 



of the Acad. N. Sc., Philadelphia, February, 1905. 



CAJAL, RAMON Y. .."Le Retine des Vertebres," La Cellule, Vol. IX, 1893. 

 DOGIEL, A. S. "Uber die nervosen Elemente in der Retina des Menchen," Arch. f. 



mik. Anat., Band XXXVIII. 



BERNARD. " Studies in the Retina," Quart. Journ. Micr. Sc., 1902 and 1903. 

 REDIKOOZEW, W. " Untersuchungen uber den Bau der Ocellen der Insekten," Zeits. f. 



Wiss. Zool., Band LXVIII, 1900. 



THE OLFACTORY AND GUSTATORY NERVE TISSUES 



The olfactory and gustatory nerve cells form a comparatively small 

 group of perceptory neurons, which are distinguished from the others by 

 the fact that they can receive impressions directly from the atoms and 

 molecules of some substances that have been dissolved in the atmosphere 



