NERVOUS SYSTEM. VERTEBRATA. 141 



small in the Platypus. This fact, which is especially note- 

 worthy in a lowly-organised mammal, is readily explained 

 by the lessened importance and value of the sense of smell 

 to an animal whose active life is chiefly aquatic. 



The optic and oculo-motor nerves and the associated 

 parts of the brain are exceedingly small, because the sense 

 of sight is of limited value to an animal which spends much 

 of its time in dark burrows. Under such circumstances 

 the sense of touch is a much more serviceable guide, and 

 hence it is not surprising to find the organs of this sense 

 highly developed. The chief tactile organ is the broadly 

 expanded snout, covered with an extremely delicate soft 

 skin. The latter is most richly supplied with numerous 

 branches of the trigeminal nerve (see specimens 1380 H 

 and 1380 I [0. C.]), which terminate in peculiarly modified 

 end-organs (Wilson & Martin, Proc. Linn. Soc. N.S.W. 

 1895, p. 660). 



The enormous development of the fifth nerves exercises 

 a most marked influence upon the configuration of the 

 region of the pons Varolii and medulla oblongata. The 

 sensory nerves coming from the snout are inserted (wholly 

 in front of the pons) as two large strands on each side a 

 mesial nerve coming from the maxillary region (representing 

 the conjoint ophthalmic and superior maxillary divisions of 

 Human Anatomy), and a lateral nerve from the mandibular 

 region. At their insertion a huge mass of nerve-cells is 

 developed as a receptive organ, so as to produce a great 

 swelling, the tuberculum quinti , upon each side of the 

 ventral surface of the hind-brain (fig. 35). The small 

 narrow band of pontine fibres is stretched across this pro- 

 minence at the situation of its greatest breadth. In the 

 caudal direction, the tuberculum quinti tapers to a point 

 upon the lateral aspect of the upper part of the spinal cord. 

 The post-pontal part of the tuberculum quinti corresponds 

 to the tubercle of Rolando in the brain of other mammals. 



The motor root of the fifth nerve emerges on the deep 

 aspect of the cephalic projection of the tuberculum quinti 

 and, after extending around the mesial edge of the latter, 

 extends transversely outward in front of the pons to join 

 the mandibular root, Owen committed the peculiar error 



