XV.] GENERAL CHARACTERS. 243 



the middle of the lower end, between the condyles, and 

 above the thickened articular border, the bone is very thin, 

 having a hollow both in front and behind. The latter, 

 which is the deepest, is called the anconeal fossa, as it 

 receives a projecting part of the anconeal process, or ole- 

 cranon, of the ulna, when the fore-arm is fully extended. 

 It often happens that these two fossae are so deep that they 

 meet, and there is in consequence a vacuity in the bone 

 called the supi-atrochlear or intercondylar foi-amen. There is 

 usually a prominent ridge running upwards for some distance 

 on the shaft from the external condyle, called the edocondylar 

 or supinator ridge [sr), which affords a wide surface of origin 

 for the supinator muscles of the fore-arm. When much 

 developed this ridge terminates above at the groove for the 

 passage of the musculo-spinal nerve. When the edge of the 

 bone above the inner condyle is much developed, it is some- 

 times grooved, but more often obliquely perforated from 

 above, downwards and forwards, by a supracondylar foramen 

 {cf), through which the median nerve and brachial artery 

 may pass. Lastly, somewhere towards the anterior sur- 

 face of the middle of the shaft, on the outer side, there is 

 usually a roughened, elevated, longitudinal ridge {dr), some- 

 times developed into a tuberosity, for the insertion of the 

 deltoid muscle, and hence called the deltoid ridge. The 

 lower end of this ridge is separated from the upper end of 

 the supinator ridge by a wide and shallow groove, wind- 

 ing in a spiral manner downwards and forwards round the 

 outer side of the shaft of the bone, and indicating the 

 course of the musculo-spiral nerve. 



The whole of the shaft of the humerus is. developed 

 ectosteally from a smgle centre of ossification, but at each 

 extremity there is a large epiphysis : the upper one in- 

 cludes the head and both tuberosities, and is usually formed 



R 2 



