54 THE ANATOMY OF VERTEBRATED ANIMAIiJ. 



In Bats, a line drawn from trie acetabulam to the foot is 

 also, in the natural position, nearly parallel with the long axis 

 of the body. But, in attaining this position, the leg is bent 

 at the knee and turned backward ; the proper dorsal surface 

 of the thigh looking upward and forward, while the corre- 

 sponding surface of the leg looks backward and upward, and 

 the ungual phalanges are turned backward. 



The chief modifications of the manus and pes arise from 

 tlie excess, or defect, in the development of particular digits, 

 and from the manner in which the digits are connected with 

 one another, and with the carpus or tarsus. In the Ichthyo- 

 sauria and Plesiosauria, the Turtles, the Getacea and Sirenia^ 

 and, in a less degree, in the Seals, the digits are bound together 

 and cased in a common sheath of integument, so as to form 

 paddles^ in which the several digits have little or no motion 

 on one another. 



The fourth digit of the manus in the Ptzrosauria^ and the 

 four ulnar digits in tlie Bats, are vastly elongated, to support 

 the web which enables these animals to fly. In existing 

 birds the two ulnar, or post-axial, digits are aborted, the 

 metacarpals of the second and third are anchylosed together, 

 and the diiyits themselves are enclosed in a common inteofu- 

 mentary sheath ; the third invariably, and the second usually, 

 is devoid of a claw. The metacarpal of the pollex is anchy- 

 losed with the others, but the rest of that digit is free, and 

 frequently provided with a claw. 



Amono^ terrestrial mammals, the most strikino: chano-es of 

 the manus and pes arise from the gradual reduction in the 

 number of the perfect digits from the normal number of five 

 to four [Sus), three {Jihinoceros), two (most Humi/iantia)^ or 

 one [E^uidoe). 



The Pectorcd and Pehnc Arches. — The proximal skeletal 

 elements of each pair of limbs {humeri or femora) are sup- 

 ported by a primitively cartilaginous, ^;ec^o?'a^, or. pelvic 

 girdle, which lies external to the costal elements of the verte- 

 bral skeleton. This girdle may consist of a simple cartilagi- 

 nous arc (as in the Sharks and Kays), or it may be complicated 

 by subdivisions and additions. 



The pectoral arch may be connected with tlie skull, or 

 with the vertebral column, by muscles, ligaments, or dermal 

 ossifications, though, primitively, it is perfectly free from, and 

 independent of, both ; but it is never united with the verte-^ 

 br;je by the intermediation of i-ibs. At first, it consists of one 



