290 



AVES. 



laws of co-existence and all that relates to the 

 nature of the organs and their essential func- 

 tions, as at the present day. They afford no 

 evidence, not even a trace of any part having 

 been lengthened or curtailed, or otherwise pro- 

 gressively modified, either by the operation of 

 external causes or by internal voluntary im- 

 pulse. 



Myology. The muscular system of Birds 

 is remarkable for the distinctness and density 

 of the carneous fibres, their deep red colour, 

 and their marked separation from the ten- 

 dons, which are of a brilliant shining colour, 

 and have a peculiar tendency to ossification. 

 This high degree of development results from 

 the rapid circulation of very warm blood, 

 which is highly oxygenated in consequence of 

 the activity and extent of the respiratory func- 

 tion. The energy of the muscular contraction 

 in this class is in the ratio of the activity of 

 the vital functions, but its permanent irrita- 

 bility is proportionally low, as Carus has justly 

 observed. 



Fig. 133. 



Muscles of a 



These characteristic properties are mani- 

 fested in the greatest degree in the muscles 

 of those families of the Insessores which take 



their food on the wing, as the Hirundinida and 

 Trochilida (Swallows and Humming-birds) ; 

 in the Diurnal Raptores and in the long- 

 winged Palmipedes, as the Albatross, Tropic 

 Bird, &c. In the more heavy and slow- 

 moving Herbivorous families, and in the short- 

 winged Swimmers, as the Penguins, &c. the 

 muscles resemble those of the Reptilia in their 

 softness and pale-colour. 



The mechanical disposition of the muscular 

 system is admirably adapted to the aerial loco- 

 motion of this class; the principal masses 

 being collected below the centre of gravity, 

 beneath the stev.ium, beneath the pelvis, and 

 upon the thighs, they act like the ballast of a 

 vessel and assist in maintaining the steadiness 

 of the body during flight, while at the same 

 time the extremities require only long and thin 

 tendons for the communication of the muscu- 

 lar influence to them and are thereby rendered 

 light and slender. 



Muscles of the trunk. The muscles of the 

 cervical region are the most developed, as might 

 be expected from the size and mobility of this 

 part of the spine ; the muscles which are situ- 

 ated on the dorsal and lumbar regions are, on 

 the other hand, very indistinct, feeble, and but 

 slightly carneous ; they are not, however, 

 entirely wanting. 



The Semi-spinalis dorsi or Opisthotenar, is 

 easily recognizable, occupying the space be- 

 tween the spinous and transverse processes, 

 arising from the anterior margin of the ilium 

 and the transverse processes of the sacrum, 

 and attached by means of long tendons to the 

 transverse processes of the costal vertebrae. 

 It is most developed in those birds which have 

 the greatest mobility in this part of the spine, 

 as in the Penguins, in which the external venter 

 of the muscle is well developed, inserted into 

 the vertebral ends of the ribs, and adapted to 

 support the body in the erect position which 

 these birds assume while standing. 



On the mesial aspect of this muscle and 

 somewhat covered by it, the Spinalis dorsi may 

 be distinctly traced, passing from the spinous 

 processes behind, to those at the anterior part 

 of the trunk and beginning of the neck. 



The Cervicalis ascendens (1, Jig. 133) is the 

 chief extensor of the neck : it rises from the 

 spines of the anterior dorsal vertebrae, and is 

 inserted by long and separate fasciculi into the 

 posterior articular processes of the second, 

 third, and fourth cervical vertebrae. In this 

 course it receives descending slips of muscle 

 from the spines of the inferior cervical vertebrae, 

 and ascending fasciculi, which furnish tendons 

 to the fifth and sixth vertebrae, and to the atlas, 

 so that it is enabled to extend the neck even 

 while the head is raised. 



Muscles corresponding to the Intertrans- 

 versales (2) are continued on the neck from 

 the external belly of the Opisthotenar ; these 

 slips extend from the articular processes of the 

 dorsal vertebrae to those of the inferior cervical. 

 Posterior to the Intertransversales, the Semispi- 

 nalis colli (3) is seen passing from the trans- 

 verse to the spinous processes. 

 The Longus colli arises from the anterior 



