OF URCHINS IN GENERAL. 235 



prolonged laterally, and unites with the great or- 

 bicular cutaneus, towards the neck. 



" These are the muscles of the superficial layer : 

 there are still some others, which are appendices 

 of the former, and are situated under the muscles 

 of the back. One arises from the head, where it is 

 attached, on both sides, to the posterior edge of 

 the external meatus auditorius. It is lost poste- 

 riorly in the anterior point of the orbicular mus- 

 cle. Another small bundle of fleshy fibres arises 

 from the last cervical processes, and is lost in the 

 cutaneus of the back. Lastly, below the great 

 orbicular muscle, we observe some transverse 

 fibres, which form a very thin layer. The anterior 

 are attached to the internal and upper part of the 

 humerus: the posterior, to the external bundle of 

 the third neutral portion. 



" Let us now consider the use of these muscles. 



" The animal, when rolled up like a ball, is 

 enveloped by the orbicular muscle. To preserve 

 this position, it is sufficient to contract the mar- 

 ginal fibres, which are very strong, and which, in 

 closing the purse so as to cover the belly, have the 

 effect of a sphincter. When the animal wishes to 

 return to its ordinary posture, it unrolls itself thus: 

 The middle fibres of the oval contract; the Exter- 

 nal fibres at first relax, and leave the belly and the 

 feet free; all the circular fibres then contract toge- 

 ther, and gather up towards the back. By this 

 general contraction the accessary muscles are ren- 

 dered 



