SKELETON. NERVOUS SYSTEM. VISCERA. 643 



supply of nerves and a remarkably delicate sense of touch, 

 the surface of the body is closely covered with hair. 



The skeleton (Fig. 326) is remarkably light, and the bones 

 of the limbs have large medullary cavities. It is distinguished 

 by the large size and rigidity of the thoracic framework, by the 

 possession of a keel on the presternum, by the length of the 

 strongly developed sacrum with which the ischia are united, 

 and by the fact that the anterior limbs are larger than the 

 posterior, and possess four greatly elongated digits (digits 2-5). 

 Moreover the hind-limb has been rotated outwards so that the 

 knee is directed backwards and the sole of the foot forwards, 

 the hallux being external. The calcaneum carries a bony or 

 cartilaginous process, the calcar, which projects backwards 

 and inwards supporting the interfemoral part of the patagium. 

 The cerebral hemispheres are smooth and do not extend over 

 the cerebellum. The spinal cord is reduced to a fine thread 

 in the lumbar-sacral region. The eyes are relatively small 

 but the senses of hearing and of touch, and probably of smell, 

 are enormously developed. Spallanzani has shown that bats 

 which have been made blind are able to avoid all obstacles, 

 such as strings stretched across a room, in their flight. The 

 pinna of the ear is always large, and often of great size ; it 

 probably not only intensifies the power of hearing, but also 

 acts as an organ of touch in the species in which the nose-leaf, 

 which is probably the tactile organ par excellence, is not 

 present. 



The dentition never exceeds i f c } p ;} m ij, and is frequently 

 reduced from this. The alimentary canal is distinguished by 

 the narrowness of the oesophagus, the relatively short length 

 of the intestine and by the general absence of a caecum. 



The heart and lungs are proportionately large. Both 

 superior venae cavae are present. 



In the reproductive system, the penis is pendent and often 

 provided with a bone, the testes are abdominal, but descend 

 into the perinaeum in the breeding season, and the clitoris is 

 occasionally perforated by the urethra. The uterus may be 

 rounded or bicornuate, and the placenta is discoidal and deci- 

 duate. They bear one or two young at a birth, and carry them 

 about with them during their flight. 



Bats are nocturnal animals. Some of them are insectivorous, 



