292 



AVES. 



and into the sides of the lower jaw from the 

 articulation as far forward as the commence- 

 ment of the horny bill. 



In the Cormorant there projects backwards 

 from the spine or squamous element of the 

 occipital bone, an osseous style about an inch 

 in length, of a trihedral figure and tapering to 

 a point. It is not anchylosed as a process of 

 the occiput, but is moveably articulated to it ; 

 and its description has been referred to this 

 section because it does not constitute a regular 

 part of the skeleton, not representing any 

 essential element of the bony fabric, but is to 

 be regarded like the bony tendons of the legs 

 as an ossification of the intermuscular aponeu- 

 rosis of the temporal muscles to which it 

 affords a more extensive and firmer origin. 

 This, indeed, is its essential use,* for the mus- 

 cles of the upper part of the neck are inserted 

 into the occipital bone, and glide beneath the 

 posterior or superadded fasciculi of the tem- 

 poral muscle. Analogous parts appended to 

 the true spinous processes of the vertebrae are 

 met with abundantly in the inferior vertebrate 

 classes, especially in fishes, where they extend 

 frequently above the spines of the whole ver- 

 tebral column, increasing the surface of origin 

 of the lateral series of muscles. 



The muscle analogous to the Biventer 

 maxilla: (18) arises by two portions, the one 

 from the lateral depression of the occiput, the 

 other from the depression behind and below 

 the external meatus auditorius ; they are in- 

 serted into the back part and angle of 

 the lower jaw. A similar disposition of 

 the dignstricus is met with in many of the 

 mammalia; even in the Orang-utan (Simla 

 Satyrus) it is equally devoid of a central 

 tendon, and is unconnected with the os hyoides. 



The openers and closers of the mandibles 

 present very slight differences of bulk in rela- 

 tion to the development of the parts they are 

 destined to move; their disproportion to the 

 bill is, on the contrary, truly remarkable in the 

 Horn-bills, Toucans, and Pelican, and the bill 

 is but weakly closed in these in comparison 

 with the shorter-billed birds. 



The upper mandible is moved by three 

 muscles on either side. The first is of a radiated 

 form, arises from the septum of the orbits, and 

 converges to be inserted into the external and 

 posterior end of the pterygoid bone, just where 

 this is articulated to the tympanic bone. It 

 draws forward the pterygoid bone, which pushes 

 against and raises the upper jaw. 



The second muscle analogous to the External 

 Pterygoid arises from the space between the 

 posterior part of the orbit and external meatus 

 auditorius, and is inserted into the internal 

 process and contiguous surface of the tympanic 

 bone ; it affects the pterygoid process, and con- 

 sequently the upper mandible in the same 

 way as the preceding muscles, and assists in 

 opening the bill. 



The Pterygoideus Internus is a long and 



* See Yarrell ' On the Anatomy of the Cormo- 

 ant/ Zool. Trans, v. iv. p. 235. 



slender muscle ; it arises from the pterygoid 

 process and body of the sphenoid, and is in- 

 serted principally into the inner side of the 

 lower jaw and tympanic bone ; it also sends 

 off a small tendon to the membrane of the 

 palate. This muscle draws forward the lower 

 jaw and depresses the upper one. 



In the Cross-bill ( Loxia curvirostra) there 

 is a remarkable want of symmetry in the 

 muscles of the jaws on the two sides of the 

 head corresponding to their peculiar position. 

 Those of the side towards which the lower 

 jaw is drawn in a state of rest (which varies 

 in different individuals) are most developed, 

 and act upon the mandibles with a force that 

 enables the bird to dislodge the seeds of the 

 fir-cones, which constitute its food. 



The articulation of the lower jaw is strength- 

 ened and its movements restrained by two 

 strong ligaments, one of these (a) is extended 

 from the ligament completing the lower part 

 of the orbit, or from the zygomatic proce ss of 

 the temporal bone, and is inserted at the outer 

 protuberance near the joint of the lower jaw, 

 and must prevent the bill from being too 

 widely opened. The second ligament extends 

 from the zygomatic process of the temporal bone 

 directly backwards to the posterior part of the 

 articular depression of the lower jaw, and is 

 designed to guard against the backward dislo- 

 cation of the lower jaw. 



The muscles of the ribs. The levatores 

 costarum arise from the posterior part of the 

 extremities of the transverse processes, and 

 converge to be inserted into the anterior 

 margin of the succeeding posterior rib. Those 

 of the first and second ribs represent the 

 Scalcni, and are of larger size, arising from 

 the last and penultimate cervical vertebrae. 



The Intercostales externi appear to be con- 

 tinuations of the Levatores costarum, and are 

 usually divided into an anterior and posterior 

 moiety corresponding to the marked separation 

 and moveable articulation between the vertebral 

 and sternal portions of the ribs ; the anterior 

 division arises from the costal appendage and 

 extends to the anterior extremity of the rib ; 

 to afford a more advantageous origin to this 

 inspiratory muscle would appear, therefore, to 

 be one of the uses of the costal appendages, 

 as well as to strengthen the connection of the 

 ribs to each other. 



The Internal intercostal* commence at the 

 sternal extremities of the ribs, as in mammalia, 

 but extend backwards no farther than the costal 

 appendages; their fibres run in an opposite 

 direction to the external intercostals, and are 

 shorter, the insertion into the posterior suc- 

 ceeding rib being by a thin but wide aponeu- 

 rosis : in the Penguin they are, however, 

 wholly muscular. Two other layers of inter- 

 costal muscles, corresponding to the triangu- 

 laris sterni, and having the same direction 

 of fibres, are extended from before backwards 

 and outwards to the four anterior sternal por- 

 tions of the ribs ; arising from the superior and 

 external angle of the sternum. 



The muscles of the abdomen are small and 



