SEC. IV ANATOMY OF BIRDS 287 



trivances for the mechanical advantage of the muscle in flexing and 

 extending this mobile part of the body. Muscles of the hyoidean 

 apparatus acquire a singular development in woodpeckers. The 

 lower jaw is depressed particularly by muscle inserted into the end 

 of the mandible ; the upper is elevated by particular muscles operat- 

 ing the pterygoid and quadrate bones. Temporal, masseteric, and 

 ordinary pterygoid muscles close the jaws. They are unsymmetrical 

 in Lmia. 



The diaphragm, the musculo-membranous partition which in 

 mammals divides the thoracic from the abdominal cavity, is only 

 represented in birds in a rudimentary condition. Macgillivray has 

 figured that of the rook as consisting of three fleshy slips, v, v, v, 

 passing from as many ribs, 4, 5, 6, to the pleural sac of the lungs, 

 t, t, in Fig. 101. It is best developed in the Apteryx. 



The remarkable specialisation of both limbs, — the former for 

 flight, the latter for the perfectly bipedal locomotion which only 

 birds besides man enjoy, — results in corresponding peculiarities of 

 the muscular mechanism. Muscles beyond the shoulder are greatly 

 reduced in number and complexity from an ordinary quadrupedal 

 standard ; those of the legs are rather increased, and their configura- 

 tion, relative size, and to some extent their relations, are so much 

 changed, that great difficulty is experienced in identifying them 

 with the corresponding muscles of quadrupeds. The result is 

 great confusion in their nomenclature, which is still shifting, though 

 much has been done of late to give it precision. Attention has 

 recently been called by Garrod to the classificatory value of certain 

 muscles of the limbs. The tensor patagii, that muscle or those 

 muscles which may have elastic tendons, and by which the folds of 

 skin in the angles of the wing-bones are regulated, may have 

 different characters in different groups of birds. It has long been 

 known that particular muscles of the hind limb are in direct and 

 important relation to the prehensile power of the toes, and conse- 

 quently co-ordinated with the insessorial or the reverse character of 

 the foot. In the highest birds, Passeres, the foot grasps with great 

 facility, owing to the distinctness or individuality of the Jlexor longus 

 hallucis, or bender of the hind toe. The ambiens (Lat. ambiens, 

 going around) is a muscle of which Garrod has even made so much 

 as to divide all birds into two primary groups according to whether 

 they possess it or not. The ambiens arises from the pelvis about 

 the acetabulum, and passes along the inner side of the thigh ; its 

 tendon runs over the convexity of the knee to the outer side, and 

 ends by connecting with the Jlexor digitorum perforatus — one of the 

 muscles which bend the toes collectively. When this arrangement 

 obtains, the result is that when a bird goes to roost, and squats on 

 its perch, the toes automatically clasp the perch by the strain upon 



