214 NATURAL HISTORY. 



number, and the palm-boues five, while the number of small bones in the fingers varies a good deal, but 

 the number of fingers is five. In most birds all these numbers are reduced. Just beyond the fore 

 arm, the larger bone of which has often small projections indicating the points at which the secondary 

 feathers have been attached, there are two small bones (f), then comes a longer bone (y k i), as it 

 seems, in which there is an elongated space. Now, this bone consists of three metacarpals and one 

 wrist-bone ; the two outer metacarpals are absent, the two innermost ones have completely united 

 with one another, and with the (true) middle metacarpal bone at their upper end ; while the second 

 and third metacarpals are also united at the other or finger end. The inner digit (k), or that which 

 corresponds to man's thumb, has two joints (j)halanges), and may be clawed ; the next has three joints, 

 and may also be clawed ; while the third finger, which has never more than two joints, is never 

 known to carry a claw. In the Archceopteryx the metacai-pal bones are well developed, and are 

 not, as in recent birds, united together. No idea of a bird's flying powers can be fairly gathered 

 from the length of the hand, for it is long in Swifts and short in Albatrosses, for example ; although 

 it is to be noted that in the former the single bone (humerus) of the arm is short, and in the latter 

 long. 



As in the breast-girdle, the bones of the hinder or hip-girdle, by which the hind-limbs are con- 

 nected with the body, are three in number ; of these the upper one is greatly flattened out and projects 

 very far forwards, thus aiding in the formation of the firm back of flying birds ; the other two bones 

 are much more slender, and are directed backwards and downwards. It is a curious circumstance that 

 it is in one bird only, in either case, that these bones are directly connected at their lower ends with 

 their fellow on the opposite side ; those which are known as the j)ubes (pb) are so in the African Ostrich, 

 and those which are known as the ischia (is) in the Rhea of South America. These two bones, with 

 the large, flat ilia (il), take part in forming the cavity in which the head of the thigh-bone plays ; the 

 outer of the two bones (ft) which are found in the leg is rarely as long as, and is always much more 

 slender than the other (ti), which has a strong ridge on its front face. There is yet another very remark- 

 able point of resemblance between birds and reptiles, in that the " ankle-joint " is in both cases 

 situated between the two rows of bones which make up the " ankle " (tarsus). In birds this arrange- 

 ment is carried to a still further extent, for the single bone of the upper row is early united with the 

 shin-bone, as may be seen under those unfortunate circumstances in which the poulterer has provided 

 an aged fowl (aged, that is, for eating) ; in more fortunate cases it will be found possible to separate a 

 small bone from the lower end of the shin-bone of the leg. 



In no case does any bird, even A rchceopteryx, possess a fifth toe. Unlike mammals, the number of 

 joints in the toes varies greatly in birds. In those which possess four toes we find the following 

 number of joints : in the first, two ; in the second, three ; in the third, four ; and in the fourth, five. 

 This rule holds for nearly all birds, but the Swifts have never more than three joints, and in the 

 Goat-sucker and the Sand Grouse there are two less than ordinary on the fourth toe. In a number 

 of birds the inner toe (big toe of man) disappears, and in the Ostrich proper the next division of the 

 "typically" five-toed foot, or second toe, has no toe-joints. 



In dealing with the muscular system of birds, we need here concern ourselves with only those 

 special muscles which are modified in accordance with the necessities of the bird's habits, and 

 those other muscles which have been brought into special notice by valuable investigations. 



That great fleshy mass which is found on the breast of a bird, and which is not unknown 

 to those who are fond of a good " dish," consists of three separate muscles, two of which depress, 

 while the other elevates the wing. The presence of the elevator muscle on the lower side of the 

 sternum is a curious arrangement by which the centre of gravity of the animal is lowered a 

 most necessary condition in flight ; the tendon from this muscle passes through a pulley-like canal 

 to be inserted into the upper side of the head of the bone, which, as has already been explained, 

 is known as the humerus, so that when it contracts it draws this bone up. The ability of the 

 wings to resist the pressure of the air is clearly dependent on the power possessed by these muscles. 

 Borelli has calculated that the " pectoral muscles " of the bird exceed in weight all the other muscles 

 taken together, whilst in man the pectoral muscles are but a seventieth part of the mass of the 

 muscles. 



The large and important muscles, which in the Mammalia constitute the diaphragm, or 



