RODENTIA AVES. 269 



The longissimus dorsi is attached to the mammillary processes of the lumbar 

 vertebrae. The muscles of mastication are also well developed, especually the 

 masseter. 



The muscles of the limbs bear a general resemblance to those of the dog. 

 In the fore-arm there is no rotation, hence we find the supinatur lotujus and 

 jrronatur quadratns wanting ; the other rotators being small, and acting as 

 mere flexors of the elbow joint. 



AVES. 



A remarkable feature in the muscular system of birds is the liability of the 

 tendons to ossification, and this condition is by no means peculiar to old 

 animals. 



Comparing, in the wide sense, the muscles of a bird with those of a mammal, 

 we are struck with their wonderful morphological similarity, altliough the 

 peculiarities of the skeleton, and physiological characteristics of the animal 

 lead to various deviations in their form and development. 



The muscles of the pectoral limbs, and notably those of the pectoral group, 

 are the muscles of flight. There are two pectoral muscles, Avhich may be 

 called the magnus and parvus. The rnagnus, an enormous muscle, is the 

 depressor of the wing (depressor alas) ; it is placed posteriorly, arising from 

 the whole of the sternum, carina, and last ribs, and being inserted to the 

 humerus near its head. The pedoralis ijarvus (levator ate) is small, and 

 situated in the depression between the clavicle and coracoid bone, its tendon 

 of insertion passing inside and over the neck of the scapula, as over a pulley, 

 and becoming inserted to the humerus ; this is the levator of the wing. 



A peculiar arrangement of the muscles enables the perching birds to '-roost.'* 

 A kind of ligament passes over the front of the knee joint and joins the flexor 

 tendon, so that when the tibia is flexed on the femur, the digits are also flexed. 

 Hence during sleep, when the weight of the animal flexes the knee-joint, simul- 

 taneously it causes the digits to clasp the perch. 



The diaphragm in birds is said to assist both in the dilatation of the lungs,, 

 and of the air sacs. Some authorities, however, dispute its existence, others- 

 again describe it as rudimentary. 



