196 



COMPARATIVE ANATOMY 



in the skin of Birds and Mammals, and in the former they are 

 particularly abundant on the beak and at the bases of the con- 

 tour-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. the various organs 

 of the abdominal cavity, the con- 

 junctiva, the fasciiB, tendons, liga- 

 ments, vas deferens, periosteum, peri- 

 cardium, pleura, corpus cavernosum 

 and spongiosum, the wing-membrane 

 of Bats, &c.). 



The tactile cells and tactile and 

 club-shaped corpuscles are all con- 

 cerned with the sense of touch. 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 pro- 

 vided with varicose swellings which 

 end freely in the epidermis, are con- 

 cerned in this process. Such free 

 nerve- endings occur in the skin of 

 all Vertebrates, and consist of an 

 intercellular network, no direct con- 

 nection between nerve and epithelial 

 cell having been observed. 



Pig. 155. 



-A Pacinian 



PtJSCLE. 



COE- 



A, axis fibre ; A^, tufted or knob- 

 like end of the same ; NS, nuc- 

 leated sheath of nerve, which 

 passes into the external longi- 

 tudinal series of lamellfe, L ; Q, 

 internal, circular layer of the 

 external part of the club ; JK, 

 internal part of the club formed 

 of the cell-pillars. 



OLFACTOEY ORGAN. 



The olfactory lobe as already mentioned (p. 153) represents a pro- 

 longation of the secondary fore-brain, the ventricle of which is tem- 

 porarily or permanently continued into it. In some cases it becomes 

 differentiated into olfactory bulb, tract, and tubercle (pp. 159-17-5). 



The olfactory nerves proper are connected with the bulb, and are 

 usually arranged in a single bundle on either side, with more or less 

 distinct indications of a subdivision into two bundles : they ap- 

 parently arise in continuity with the epithelium of the nasal 

 involution (comp, pp. 177, 187) and then grow centripetally, 

 uniting with the olfactory lobe or bulb secondarily. 



In all Mammals but Ornithorhynchus, as well as in Menopoma, 

 Apteryx, and the extinct Dinornis, the olfactory nerves pass into 

 this nasal cavity separately, through a cribriform plate of the 

 •ethmoid (p. 99). 



