258 COMPARATIVE ANATOMY 



by numerous lamellae : on it either knob : like buds or networks can 

 be recognised, the latter surrounding the main fibre (Fig. 188). 

 Often also there is a double column of cells surrounding the axis. 



Organs of this kind are universally present in the deeper la} vrs 

 of the derm in Birds and Mammals : in the former they arc 

 particularly abundant on the beak and at the bases of the 

 contour- feathers of the wings* and tail, and are also found on 

 the tongue. They occur, moreover, in various other regions, both 

 in Birds and Mammals (e.g. on the various organs of the ab- 

 dominal cavity, the conjunctiva, the fasciae, tendons, ligaments, 

 the vas deferens, periosteum, submaxillary glands, mesentery, peri- 

 cardium, pleura, corpus cavernosum and spongiosum, the wing- 

 membrane of Bats, &c.). Their size varies greatly even in the same 

 individual. 



The tactile cells and tactile and club-shaped corpuscles are all 

 concerned with the sense of touch and pressure. It is impossible to 

 say definitely what nerve-endings have to do with the perception 

 of temperature ; it is not improbable that the touch-cells, as well as 

 the nerve-fibres often provided with varicose swellings which end 

 freely in the epiderm, are here concerned. Such free nerve-endings 

 occur in the skin of all Craniata and consist of a branched, inter- 

 cellular network, no direct connection between nerve and epithelial 

 cell having been observed. 



OLFACTORY ORGAN. 



The olfactory nerves are connected with the olfactory lobes 

 which arise as prolongations of the secondary fore-brain, the 

 ventricle of which is temporarily or permanently continued into 

 them : as already mentioned, each olfactory lobe is, in some cases, 

 differentiated into bulb, tract, and tubercle. 



The filaments of the olfactory nerves are arranged in a single 

 bundle on either side, or in two more or less distinct bundles. 

 The individual fibres pass from the cranial cavity into the nasal 

 cavity either separately, through a cribriform plate of the ethmoid 

 (p. 129), or through a single aperture on either side. The latter 

 is the case, e.g. in Fishes, all Amphibians except Menopoma, 

 Reptiles, all Birds except Apteryx and the extinct Dinornis, and 

 Ornithorhynchus : in all Mammals except the last mentioned a 

 cribriform plate is present. 



The primary origin of the olfactory organ is by no means 

 understood, and it is doubtful whether it can be said to have a true 

 olfactory function in aquatic types. In its simplest form, the organ 

 consists of a ventral, paired, pit-like depression of the integument 

 of the snout opening on to the surface by an external nostril. It 

 is lined by ectodermal epithelium, which gives rise to a " primary 

 olfactory ganglion," the individual elements of which at first re- 

 semble unipolar nerve-cells : from these, the olfactory fibres grow 



