96 



ANIMAL ORGANIZATION. 



branes, the one enveloping the other. Fig. 36 shows the 

 bulb laid open by dividing the membranes and turning them 

 aside. The horny portion of the quill is secreted by the 



internal membrane, i, and deposited in successive laminae. 

 The external membrane is seen at o. The pulp itself, seen at 

 p, is still more curiously organized; its surface being fluted, 

 or formed into longitudinal processes. The horny matter, 

 being deposited on these processes, is moulded to their shape, 

 and concretes into laminae which converge from the circum- 

 ference of the cylinder towards the centre. The section 

 (Fig. 36) shows these converging laminae, which being of a 

 dark colour, give to the surface of the quill the appearance 

 of being grooved; this, however, is merely an optical illu- 

 sion occasioned by the dark laminae being seen through the 

 transparent exterior covering; as may readily be detected 

 by viewing the surface with a magnifying glass.* After a 

 certain period of the growth of the quill, the pulp ceases to 

 supply the materials for forming the spongy substance which 

 occupies the interior of the quill. But although it no longer 

 secretes, it still retains Its place; and the capsule continuing 

 to deposite horn, the quill becomes a hollow tube of consi- 

 derable diameter. When it has attained a certain size, the 



• It is observed by F. Cuvler, that this striated appearance is peculiar to 

 the quills of porcupines of the old world. Those from America have no 

 such arrang-ement of laminae. 



