MAMMALS. 601 



With the nasal cavity is connected in different mammals, esj)e- 

 cially the ruminants and rodents, an organ discovered \>j the 

 Danish anatomist Jacobson, and tln-ongh which a closer connec- 

 tion perhaps is established between taste and smell. It consists of 

 two longitudinal sacs, of which one is placed on each side of the 

 cartilaginous septum of the nose, lying upon an excavation of that 

 portion of the preraaxillary by which the fore part of the palate is 

 formed. These sacs, covered internally by a glandular mucous 

 membrane, are surrounded by a cartilaginous sheath and usually 

 communicate with a duct (formerly discovered by Steno), which 

 runs through the foramina incisiva to the palate \ 



Eyes are always present in this class. In some, however, the 

 sight is deficient, for the skin, without being fissured to form 

 eyelids, passes in front of the eyeball, which in such instances is 

 always very small (as in Sjmlax, Mus typhlus, Pall.'^). The rela- 

 tive size of the eyes is on the whole less than in birds. The true 

 whales have relatively (i.e. in proportion to their extraordinarily 

 large body) very small eyes, although their eyes, regarded by 

 themselves, are the largest in the whole animal kingdom. The 

 eyeball commonly is almost spherical; the sclerotic coat is desti- 

 tute of the support of a bony ring, which we remarked formerly 

 in birds and difterent reptiles. In the true cetaceans the sclerotic 

 is of remarkable thickness, from wlience the cavity of this sense- 

 organ is of still less size than that inferred from the small relative 

 size of the eyeball. In man and the apes there is at the entrance 

 of the optic nerve at the inside a yellow spot on the retina, which 

 was first noticed by Scemmerring. In the dolphins, the greater 

 number of carnivores and the ungulates, the choroid coat is for a 

 space at the back part of the eyeball destitute of the black colour- 

 ing-matter Avhich is elsewhere dispersed over it, and its place is 



1 Compare CuviER Rapport fait a I'Insfitut, Ann. dii Miisemn, XVIII. 1811, pp. 

 412 — 424; Rosenthal in Tiedemann m. Teevieanus Zeitschr. fiir Physiol. 11. 1, 

 1827, s. 289 — 300, with a figure of this part in the sheep. In the horse also it is 

 present, but there is no duct running to the palate. In man this organ is absent, but 

 the duct (at least often) is present. The duct itself, according to a remark of HusCHKE, 

 is of no physiological significance, but is merely a vestige of a fcetal state, in which the 

 cavities of the mouth and nose were in connexion through the entirely fissured palate. 

 Lehre von den Eingetveiden , s. 612. 



2 Compare on the eyes of this animal RuDOLPHi Physiol. 11. i, s. 157, Anni. i. 



