4G6 THE MECHANICAL FUNCTIONS. 



We have seen that there is a tribe of fishes 

 armed externally with sharp spines, which they 

 are capable of erecting when in danger of attack. 

 The Porcupine and the Hedgehog-, which belong to 

 the family of insectivorons quadrupeds, are fur- 

 nished with a similar kind of defensive armour. 

 For the purpose of erecting these bristles, when 

 the animal is irritated or alarmed, there is provided 

 a peculiar set of muscular bands, which forms part 

 of the usual subcutaneous layer, termed the pamii- 

 cidus carnosns. In the hedgehog these muscles are 

 very complicated, and give the animal the power 

 of rolling itself into a ball. A minute description 

 of these muscles has been given by Cuvier, who 

 found that the whole body is enveloped in a large 

 muscular bag, or mantle, lying immediately under 

 the integuments ; and capable, by the contraction 

 of different portions of its fibres, of carrying the 

 skin over a great extent of surface. In the usual 

 state of the animal, this broad muscle appears on 

 the back (as represented in Fig. 219), contracted 



into a thick oval disk, of which the fibres are 

 much accumulated at the circumference. From 

 the edges of this disk there pass down auxiliary 

 muscles towards the lower parts of the body ; the 

 action of which muscles tends to draw the skin 

 downwards, and to coil it over the head and paws, 

 in the manner shown in Fig. 220, like the closing 

 of the mouth of a great bag. 



