SKELETON AND LIMBS. 773 



finally taking its place altogether. The thickened rings, thus solidified 

 by cartilaginous deposit, become the bodies of the vertebrae ; while their 

 transverse and articulating processes, with the laminae and spinous pro- 

 cesses, are formed by outgrowths from the bodies in various directions. 



When the union of the dorsal plates upon the median line fails to 

 take place, the spinal canal remains open at that situation, and presents 

 the malformation known as spina bifida. This may consist simply in 

 a fissure of the spinal canal, more or less extensive, in which case it 

 may sometimes be cured, or may even close spontaneously ; or it may 

 be complicated with either an imperfect development or complete absence 

 of the spinal cord at the same spot, producing permanent paralysis of 

 the lower extremities. 



The entire skeleton is at first cartilaginous. The first points of ossifi- 

 cation show themselves about the beginning of the second month, almost 

 simultaneously in the clavicle and the lower jaw. Then come, in the 

 following order, the femur, humerus, and tibia, the superior maxilla, the 

 bodies of the vertebrae, the ribs, the vault of the cranium, the scapula 

 and the pelvis, the metacarpus and metatarsus, and the phalanges of the 

 fingers and toes. The bones of the carpus are all cartilaginous at birth, 

 and do not begin to ossify until a year afterward. According to Cru- 

 veilhier, the calcaneum, the cuboid, and sometimes the astragalus, begin 

 their ossification during the latter periods of foetal life, but the remainder 

 of the tarsus is cartilaginous at birth. The lower extremity of the 

 femur, according to the same authority, shows a point of ossification at 

 birth ; all the other extremities of the long bones being still in a carti- 

 laginous condition at this time. The scaphoid bone of the tarsus and 

 the pisiform bone of the carpus are the last to commence their ossifica- 

 tion, several years after birth. Nearly all the bones ossify from several 

 distinct points ; the ossification spreading as the cartilage increases in 

 size, and the various bony pieces, thus produced, uniting with each 

 other at a later period, usually some time after birth. 



The limbs appear by a budding process, as offshoots of the external 

 blastodermic layer. They are at first mere rounded 'elevations, without 

 any separation between the fingers and toes, or any distinction between 

 the different articulations. Subsequently the free extremity of each 

 limb becomes divided into the phalanges of the fingers or toes ; and 

 afterward the articulations of the wrist and ankle, knee and elbow, 

 shoulder and hip, appear successively from below upward. 



The lower limbs in man are less rapid in development than the upper. 

 Both the legs and the pelvis are very slightly developed in the early 

 periods of growth, as compared with the spinal column, to which they 

 are attached. The inferior extremity of the spinal column, formed by 

 the sacrum and coccyx, projects at first beyond the pelvis, forming a 

 tail, which is curled forward toward the adbomen, and terminates in a 

 pointed extremity. The entire lower half of the body, with the spinal 

 column and appendages, is also twisted, from left to right ; so that the 

 pelvis is not only curled forward, but also faces at right angles to the 



