44 EVERS'S COMPARATIVE ANATOMY. 



AVES. 



A greater degree of uniformity pervades the muscular system of 

 this than any other of the vertehrated classes, yet it will be found to 

 present many peculiarities. From the rarity of the element they 

 inhabit, as well as from their rapid and long continued movements 

 through it, their muscles require a considerable degree of vital 

 energy, hence they are red, vascular, dense, and irritable in the 

 high flying and rapacious tribes, although pale, soft, and feeble in 

 those of heavier and slower habits. The fleshy portions of the 

 muscles are short and thick, whilst the tendons are long, slender, 

 dense, and often ossified ; their trunk being almost fixed, the 

 muscles of the dorsal and lumbar regions are feeble and indistinct, 

 those of the neck, on the contrary, are well developed in accordance 

 with the perfect and varied motions of this part of the spine. The 

 muscles of the abdomen are week and feeble, and the diaphragm so 

 imperfect in the centre as to allow the heart to come in contact 

 with the liver as in reptiles. Of all the muscles, none reach so 

 great a degree of development as those of the anterior extremity, 

 especially those attached to the humerus. Birds possess three pec- 

 toral muscles, an anterior, middle, and posterior: they are all 

 attached to the sternum and the proximal extremity of the humerus. 

 In birds of flight, the great pectoral often equals in weight all the 

 other muscles of the body combined. The latissimus dorsi and 

 deltoid are feebly developed, whilst the psoEe, obturator externus, 

 and quadratus lumborum, are wholly absent. The muscles of the 

 lower extremity are remarkable for their long, slender tendons, and 

 especially for the beauty and perfection of the mechanism by which 

 they support the bird when asleep on roost, without any muscular 

 action. This is accomplished by the gracilis, which, arising from 

 the pubis, descends along the inner side of the thigh, and ends in a 

 strong tendon, which passes in front of the knee-joint, and subse- 

 quently over the projection of the heel to terminate by attaching 

 itself to the outer origin of the flexor digitorum perforatus. From 

 this disposition it results, that the more the joints are bent, the 

 firmer the twig on which the bird rests, is grasped, and the heavier 

 it sleeps, the more secure it is. Every one is familiar with the 

 fact of birds generally sleeping on one leg, this is for the purpose 

 of throwing the entire weight of their body on it, and so grasping 

 the firmer, and in order to increase the effect by adding to the 

 weight of the body, some birds are in the habit of never going to 

 roost without grasping a stone, or some ponderous body in the 

 other foot. 



Flight, which is the most chracteristic mode of progression in 

 birds, is effected by the animal springing into the air ; or, where the 

 legs are so short, and the wings so Ions: that it cannot jump high 

 enough to gain the requisite space for the expansion of the wing, 

 it throws itself from some elevated point. The humerus is next 



