STRUCTURE OF BIRDS. 



45 



feathers, or, at least, are connected with their growth. I have 

 observed that at the period of moulting, and especially when the 

 tail-quills are growing, that they are very highly developed, and, 



as is well known, 

 sometimes inflame 

 and suppurate in 

 domestic birds, 

 w^hereas in birds of 

 which the moult 

 has been complet- 

 ed, I have generally 

 found them greatly 

 diminished, and 

 frequently entirely 

 shrunk. This fact, 

 analogous to that 

 of the periodical 

 enlargement of the 

 testes in birds, af- 

 fords a key to the 

 knowledge of the nature and use of the uropygial gland, which 

 has hitherto eluded the sagacity of physiologists ; for the appli- 

 cation of the oil contained in it by the bill is certainly fanciful. 



Fig. 5. The Tail of a Thrush. 



The muscles seen in this view are : 



1. The levator coccygis^ which arises from the lower extremity 

 of the sacrum, and from the sides of the coccygeal vertebrae, 

 to be inserted into the tips of their spinous processes, and into 

 the base of the last vertebra. This muscle is, in fact, rather 

 a series of small muscles of w^hich there is one for each verte- 

 bra. Its use is to elevate the tail in various degrees ; and, 

 when acting singly, to draw it obliquely to a side. It is op- 

 posed by the depressor coccygis. 



2. The quadratiis coccygis arises from the sides and trans- 

 verse processes of the coccygeal vertebrae, and is inserted into 

 the base of the ligamentous fascia investing the base of the quills, 

 its fibres curving also round the edge of the tail, and being partly 

 inserted beneath. Its action is to spread out the tail-feathers. 



