THE SKELETON AND NERVOUS SYSTEM 1211 



met with in connection with the last cervical an<l the first lunil)ar vertebra'. The 

 cervical rib has been found extending from tlie seventli cervical vertebra to the 

 sternum. As a general rule, however, it terminates anteriorly by joining the first 

 ril), or only its anterior and i)osterior ends are formed, the two parts lu-ing coimected 

 l)y a i)and of fibrous tissue. Tlie lumbar ril», when it appears, always terminates 

 by a free end in the body wall. 



UPPER LIMB 



Humerus 



The supracondylar process and foramen. — ^\']len it is present, the supra- 

 condylar j)rocess springs from the inner surface of the' humerus, a few centimetres 

 al)ove the internal condyle; it turns downwards, and is united to the humerus at a 

 lower level ))y a l)and of fibrous tissue, which is sometimes rt'placed by lione. In the 

 latter case, a distinct supracondylar foramen is formed, through which the meflian 

 nerve passes. This foramen is very common in the lower mannnals, in am})hibians, 

 reptiles, and in their fossil ancestors. 



The supratrochlear foramen. — This foramen is formed when the septum 

 between the coronoid and olecranon fossae is not developed. It is frequent in the 

 lower races of mankind, especially in the 8outh African natives and in the 

 Veddahs. It also occurs in skeletons belonging to the Stone Age, in the gorilla, the 

 orang, and the lower apes. 



The OS centrale. — The os centrale is occasionally found as a separate bone in 

 the carpus, lying in an interval between the scaphoid, the os magnum, and the 

 trapezoid and trapezium. Its cartilaginous rudiment is always present at the second 

 month of fcetal life, but in the second half of the third month it usually fuses with 

 the scaphoid, and it is only when the cartilage remains distinct and undergoes 

 separate ossification that an os centrale is present in the adult. 



The OS centrale is a normal component of the carpus of the orang; it is present 

 in most monkeys, and it is a regular carpal element in many of the lower verte- 

 brates. 



THE LOWEPv LIMB 



The third trochanter. — In about thirty per cent, of European skeletons the 

 U])j)er part of the gluteal ridge is developed into a prominent third trochanter. 

 This prominence is more rare in negroes and anthropoid apes, but it is very fre- 

 quently present in lemurs. It is well developed in many of the lower mammals, 

 and it forms a prominent projection on the femur of the hare. 



THE MUSCULAR SYSTEM 

 {A List of Abnormal Muscles tcill be found on page IfbS. ) 



THE NERVOUS SYSTE.U 



The filum terminale. — Extending from the end (tf the conus medullaris of 

 the spinal cord to the liack of the coccyx there is a thin, cord-like structure, the 

 filum terminale; it forms part of the cauda equina, and is the lower, non-functional 

 portion of the spinal cord. It does not exist before the third month of fivtal life, 

 for up to that period the spinal cord is coextensive with the spinal canal. After 



