1004 



INSECTIVORA. 



The chiasma is yery distinct, without a trace of 

 a nerve proceeding from it." 



Fig. 453. 



The organ of hearing in the mole is so con- 

 structed as to afford the greatest possible deli- 

 cacy and acuteness of this important sense, and 

 thus to counterbalance the deficiency in that of 

 sight. It would indeed appear that the mass 

 of compact bone constituting the petrous por- 

 tion of the temporal, which surrounds the laby- 

 rinth in most of the Mammalia, must more or 

 less diminish the powers of hearing, for we find 

 it deficient in many animals, which from their 

 habits require this sense to be in the greatest 

 perfection. Thus, in the mole, the semicircular 

 canals are free and visible within the cranium, 

 without any preparation; and the parietes of 

 the cochlea itself are almost as cellular and 

 loose as we find in birds. 



The ear of the mole possesses no concha ; it 

 is small in the shrews; and in the water-shrews 

 (Hydrosorex ) the external meatus is closed at 

 the will of the animal by means of the anti- 

 tragus; a provision obviously essential to its 

 aquatic habits. 



When it is considered that the sense of 

 vision is only available on the rare occasions 

 of the appearance of the mole on the surface, 

 and then only for very limited objects, and that 

 all its intimations of danger, and its only guide 

 to the opposite sex, are by means of the sense of 

 hearing, the necessity for this extraordinary deve- 

 lopement of that sense at the expense of that of 

 sight becomes obvious. And as, from the nature 

 and situation of its food and its means of pro- 

 curing it, the sense of smell is equally neces- 

 sary for effecting this object, we find that the 

 olfactory organ is also of considerable volume. 

 The structure of the nose itself is highly curious 

 and admirably suited to the habits of the ani- 



mal. The cartilages of the nose are elongated 

 into a tube or trunk, which extends far beyond 

 the osseous basis, and is supported by a very 

 delicate moveable bone, which is represented 

 in the figure of the cranium of the mole at 

 Jig. 441. It is furnished with a muscular appa- 

 ratus of considerable complexity (see fig. 446), 

 consisting of no less than four pairs of muscles, 

 which arise from above the ears, and passing 

 forwards are inserted by separate tendons into 

 the circumference of the extremity of tin; carti- 

 laginous snout. This structure is of the utmost 

 advantage to the animal in its subterranean 

 search after worms and insects. 



There is no order of Mammalia in which a 

 greater contrast is exhibited in the external 

 covering of the body than in the different 

 groups of the Insectivora. The porcupine 

 and the mouse, amongst the Rodrntia, do not 

 offer a more remarkable contrast in this respect 

 than do the two families of the Erinaceadie and 

 the Tulpida. The habits of the hedgehog de- 

 pending for its defence upon the panoply of 

 armour which it presents to its enemies, when 

 rolled up in a compact ball by the muscular 

 apparatus already described, it is furnished on 

 all the upper and lateral parts of the body 

 with hard sharp spines or quills, jp'ig. 45-1 



Fig. 454. 



is taken from a drawing by William Bell, 

 engraved in the Hunterian Catalogue, vol. iii. 

 from which also the following description is 

 borrowed: " On the cut edge of the skin 

 may be seen the roots and sockets of the quills, 

 extending to different depths from the surface, 

 according to the period of their growth : the 

 newly formed ones are lodged deep and ter- 

 minate in a broad basis, the pulp being large 

 and active, and the cavity containing it of cor- 

 responding size; but as the growth of the quill 

 proceeds, the reflected integument forming the 

 socket contracts, and gradually draws the quill 

 nearer the surface; the pulp is at the same time 

 progressively absorbed, and the base of the 



