OCR Text |
Show 1891.] ORGAN IN T H E CROCODILIA. 153 essential functional constituent of Jacobson's organ is to be regarded " as a highly specialized portion of the olfactory epithelium " l. As it exists among Reptiles it may be defined as a distinct sac lying (on each side) immediately beneath the anterior portion of the nasal chamber, and communicating with the mouth-cavity by means of a delicate duct, which arises posteriorly and passes obliquely downwards and forwards, immediately external to the vomer, to reach its anterior extremity 2. Examination of any ordinary Lizard will show that the organ itself lies immediately above and to the outer side of the vomer, and that the point of communication between its duct and the mouth lies within the embrace and at the anterior end of a fold of mucous membrane common to it and the posterior nostril. Jacobson's organ, as is well known, attains its greatest differentiation among Mammals. Its more minute structure has been worked out by Klein 3, Harvey 4, and others ; and a more general communication upon it has been recently published by Herzfeld5, in which a classification has been attempted, in accordance with the presence or absence of naso-palatine canals and with other leading features described. This organ lies, in Mammals, within the embrace of a cartilaginous sheath (jc, fig. 3) derivative of a downgrowth (ns.") of the alary cartilage of the nose (ns."), and it is in communication with the olfactory chamber by a wide orifice (e.j.) which overlies the naso-palatine (Stenson's) canal (c.s.). The main body of the organ of each side, with its surrounding cartilage, is supported upon a scroll-shaped prolongation of the premaxilla (p.m.) usually termed its palatine process (p.p.) This passes back and at the posterior extremity of the entire organ meets a special lobe of the vomer (vo.'") ; between the latter bone and the supporting apparatus of the Jacobson's organ there is invariably an intimate relationship. Comparison of the Mammal (fig. 3) with the Crocodilian Caiman niger (fig. 2) shows that there is a fundamental similarity in position between the so-called "palatine process" of the premaxilla in the former (pp.), and that which I have herein termed the palatine lobe of the vomer in the latter (vo.'"). Albrecht0, Sutton7, and Parker8 have directed attention to the fact that the so-called "palatine process "of the Mammalia may be distinct in origin from the body of the premaxilla with which it ankyloses. I have long been suspicious of a similarly distinct origin of the same for the Common Rabbit (Lepus cuniculus) here figured; and my pupil, Mr. R. H. Burne, who, at m y request, has looked into the matter, has shown me that such is the case in embryos measuring 8 centim. in length. Parker has described the vomer of certain Edentata, Insectivora, 1 Bamsay Wright, Zoolog. Anzeiger, 1883, p. 393. 2 Cf. Leydig, op. cit. pp. 96, 99, and pi. viii. 3 Quart. Journ. Micr. Sci. n. s. vol. xxi. pp. 219 & 549 (1881). 4 Ibid. vol. xxii. p. 50 (1882). 5 Zoolog. Jahrb. Bd. iii. p. 551 (1889). 0 Corresp. deutsch. anthrop. Gesellsch. Munchcn, 1883, no. 11, p. 170. 7 P. Z. S. 1881, p. 566. 8 Phil. Tr. pt. i. 1885, pp. 1-275. |