COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 233 



Chiroptera. 



Miniopterus srJireibersii ; adult. 

 Nyctophilus sp. ? ; adult. 

 Pteropus poliocephalus ; adult. 



Primates. 

 Homo sapiens ; early foetal. 



General Observations. 



In its typical form the mammalian organ of Jacobson is a specialised portion of the 

 nasal mucous membrane, situated in the anterior part of the base of the nasal septum, 

 and forming a tubular process, blind posteriorly, but opening in front in the region of 

 the naso-palatine canal. The inner wall of the tubular organ is lined with highly 

 specialised neuro-epithelium, while the outer wall is composed of ciliated columnar 

 epithelium, considerably resembling that of the nasal passage. Into the organ there 

 open a large number of mucous glands, which are situated chiefly at its posterior 

 part. Along the outer wall, which generally bulges into the lumen of the organ, 

 there run one or more blood-vessels, frequently forming a sort of plexus. On its inner 

 and under sides, at least, the organ is supported by a curved cartilaginous plate — the 

 recurrent cartilage — which is itself supported by a curved bony plate, the palatine 

 process of the premaxillary. 



While, as a rule, the organ is fairly well developed, in a number of animals it is 

 quite absent, and in others only a rudiment is present. It is difficult at present to 

 account for the remarkable differences in the degree of development of the organ in 

 different mammals, as absolutely nothing is known for certain as to its special 

 function ; still, from comparative observations, a few interesting general conclusions 

 can be arrived at. In the first place, the organ is, as a rule, better developed in the 

 lower forms than in the higher. Thus, in both the Monotremes the organ is exceedingly 

 well developed, and in all the Marsupials yet examined it is at least fairly well devel- 

 oped, while among the Primates, it is absent, according to Herzfeld, in Cercopithecus 

 and Inuus — the only old world monkeys examined — and in man it is quite rudimentary. 

 Another conclusion that may be safely arrived at is, that in large animals the organ is, 

 relatively, considerably less developed than in the smaller sorts. For example, in the 

 two species of Dasyurus, D. maculatus and D. viverrinus, though the former is about 

 twice the size of the latter, the organs are absolutely about the same size in 

 each ; the cartilaginous capsule of the organ, however, is much larger, and the 

 extra space is filled up by a great development of glandular tissue. In all forms in 

 which the organ is developed it receives the secretion from numerous mucous glands, 

 which lie chiefly towards the posterior part of the organ, and in most small animals, e.g., 

 the mouse, where it fills the greater part of cartilaginous capsule, only at that part. 

 In the larger animals, e.g., rabbit, dog, etc., where the organ only occupies a small portion 



