82 BRITISH BIRDS 



the brain lies hi the brain-case. The nerves of the body come 

 off either from the brain or the marrow, but it is not important to 

 enumerate them. They show no difference in different kinds of 

 birds. 



The Muscles. 



The muscles of a bird are what is popularly known as its flesh. 

 When the skin is removed, the bones are seen to be covered by a 

 mass of this flesh, which is of a red colour, darker in some birds 

 than in others. For instance, in a Duck the colour is a dark red ; 

 in a Pigeon, quite a pale brown. The flesh is not, however, merely 

 a thick sheet covering the bones : it can be separated into layers 

 which are themselves made up of a number of separate pieces 

 of muscle. These individual muscles are very commonly of a 

 spindle-like shape, being thickest in the middle and dwindling 

 towards both ends, where they often end in a tough substance called 

 the tendon, which has a glistening and very characteristic appear- 

 ance. All muscles are not of this form sometimes they are strap- 

 shaped ; and not all of them end in tendons. As the most important 

 act of the bird's life that depends upon its muscles is flying, it is 

 not surprising to find that the muscle which effects the downward 

 stroke of the wing is the largest. This muscle is known as the 

 great pectoral, and it is said to be almost as large as all the other 

 muscles of the body put together. The way in which a muscle 

 effects the movements of the bones to which it is attached is by con- 

 tracting. All muscles are able to contract ; they shorten, and, ac- 

 cordingly, the ends, with whatever they happen to be attached to. are 

 brought closer together. The contraction is governed by the nerves, 

 and it has been discovered that the nerves actually end in communi- 

 cation with the fibres of which the muscle is composed. This pectoral 

 muscle lies on the breast-bone, and nearly completely covers it ; in- 

 deed, only the edge of the keel appears, and a very little tract at the 

 sides. "When this muscle is dissected away another muscle, not nearly 

 so large, comes into view underneath it ; this is called the pectoralis 

 secundus, or the second pectoral. Its action is precisely the reverse 

 of that of the great pectoral : it pulls the whig up instead of down. 

 Between them, these two muscles do most of the work in flying. 

 Naturally, in the ostrich tribe, which do not fly, they are much re- 

 duced in bulk. But they are never absent altogether, even in the 



