NECTURUS MACULATUS. 433 



The limbs when in the normal resting or swimming position are directed backwards, 

 and are held in such a way that in the hind limbs the soles face inwards, while in the 

 fore limbs the dorsal surface of the manus is ventral, and the palm dorsal. This is apt to 

 cause considerable difficulty in orienting the parts, but if the action of the limbs be 

 watched in a living aninud it will be seen that this position is due, not so much to a 

 torsion, as to a swinging of the entire liml) at the shoulder. The position assumed by the 

 fore limb is the easier to understand, and in this it is evident that the limb when extended 

 forward as in walking, rests with the palm down and the dorsal surface of the manus up, 

 but that when the entire limb is swung from the shoulder so as to change its direction and 

 point backwards, the positions of palm and dorsum are necessarily reversed. In the hind 

 limb there is some tendency to counteract this by a torsion of the limb about its own axis 

 and thus in a trailing hind foot the sole is turned somewhat inward. 



Humerus. — The humerus, like the other long bones of the limbs, consists of an 

 osseous shaft and two cartilaginous epiphyses. The usual cylindrical shape is retained 

 only at the middle while the ends are both strongly flattened, and at right angles to 

 one another, the proximal end being flattened laterally, and the distal end dorso-ventrally. 

 The most prominent jjart of the proximal epiphysis is the head, which fits into the glenoid 

 fossa of the shoulder girdle and forms the characteristic ball-and-socket joint. It is sub- 

 spherical in shape, and is slightly affected by the general flattening of the entire region. 

 Its ventral face is prolonged into a sharp ridge which is continued by the osseous 

 diaphysis and forms one of the most distinctive features of the bone. This is the crista 

 ventralis (crista deltoidea of Ecker) and .serves for the insertion of most of the ventral 

 shoulder muscles. This crest is highest near the junction of cartilage and bone and 

 rapidly recedes, so that at the middle of the diaphysis no trace remains. 



The distal end, which is broadened laterally and flattened dorso-ventrally, bears an 

 extensive median furrow running around the end and up both surfaces, and dividing it 

 into two masses which may be distinguished as the external (lateral) and the internal 

 (median) condyles, although they include parts which in higher forms are distinct from 

 the condyles themselves. The external condyle is somewhat the larger and is ball-shaped, 

 fitting into the socket in the head of the radius. It also serves as a point of origin for the 

 extensor muscles of the forearm and hand. Having these two functions, it plainly corre- 

 sponds to both the external condyle and the capitulum of higher animals (epicondylus 

 medialis and eminentia capitata of the frog ; Gaupp) . 



The internal condyle is more nearly the exact homologue of the part of that name in 

 higher forms as it gives origin to the flexor muscles of forearm and hand, but does not 

 participate as directly in the formation of the elbow joint. The groove between the twcj 

 condyles lies in the greater sigmoid notch of the ulna, and the olecranon of the ulna and 



