124 MORPHOLOGICAL TYPES 



through which the sciatic artery and nerve run to the hind 

 limb. The pubis is remarkable in that it points backwards 

 and runs along the ventral border of the ischium, from which 

 it is separated by the obturator fissure (corresponding to 

 the ischio-pubic foramen, and serving for the passage of the 

 obturator nerve). Neither the pubis nor the ischium extend 

 to the middle line ; they consequently do not meet their 

 fellows of the opposite side, and have no symphyses. The 

 absence of symphyses may be correlated with the fact that 

 birds lay relatively very large hard-shelled eggs. 



The femur is short and thick ; covering the front side of its 

 lower extremity is the patella. The tibia is fused with the 

 fibula and with the proximal tarsal bones to form the tibio- 

 tarsus. The distal tarsals are fused on to the united metarsals 

 of the 2nd, 3rd, and 4th toes to form the tarso-metatarsus. 

 The 1st metatarsal is small and free ; the digital formula of the 

 phalanges is 2, 3, 4, 5, o (there being no 5th toe). The terminal 

 phalanges bear claws. The first toe is in birds usually directed 

 backwards, and it is opposable to the other digits. This 

 arrangement enables a bird to stand securely on a narrow 

 twig of a tree, with the first toe clasped round behind the twig 

 and the remainder in front of it. The joints of the toes are 

 bent by tendons which run back beneath the foot and up 

 behind the tarso-metatarsus. The more the tarsal joint is 

 bent (in squatting), the tighter these tendons are stretched, 

 and the more strongly are the toes bent. The bird can there- 

 fore ensure a tight grip on its perch without effort, and even 

 when asleep, simply by squatting. 



It is obvious that the hind limb can bend only at the knee 

 and between what were the proximal and distal rows of tarsals : 

 this extreme form of the meso- tarsal joint is the rule in birds. 



It is to be noted that the bones of the bird's skeleton are 

 very light, and that most of them are hollow ; some of these 

 spaces communicate with the air-sacs which will be described 

 in connexion with the respiratory system. 



Alimentary System. — The tongue is small and pointed, and 

 behind it the two Eustachian tubes open into the mouth by a 

 single common aperture. The glottis in the floor of the gullet 



