276 Studies of tlic Development of tlie Human Skeleton 



in man. He showed that in young embryos the cervical region is rela- 

 tively longer than the lumbar region but that as growth proceeds there 

 is a constant proportional increase in length of the latter over the former. 

 Taking the cervical region as 100, for instance, he found that in embryos 

 below 10 mm. in length the lumbar region equals 69, while in the adult 

 it is equivalent to 150. Thus, too, while from infancy to maturity the 

 spinal column increases three and one-half times in length and the thor- 

 acic region at about the same relative ratio, the lumbar region increases 

 four times in length and the cervical but three. Other investigators, 

 including Ballantyne, 92, and Moser, 89, have in general confirmed these 

 results. Of those who have studied the proportional length of the various 

 regions in the adult Eavenel, 77, and Tenchini, 94, have made note- 

 worthy contributions. 



The post-natal lengthening of the lumbar region is associated with 

 those changes in the lumbo-sacral curve which accompany the assumption 

 of an erect posture during early childhood. Do similar alterations in 

 relative regional length accompany the straightening of the spinal column 

 which takes place during the first three months of embryonic develop- 

 ment? 



In Fig. 44, Plate X, I have represented by curved lines the vertebral 

 columns of several embryos of this period and an adult column. The 

 cervical, lumbar and coccygeal regions are represented by heavy, the 

 thoracic and sacral regions by light lines. The 5th, 6th and 7th thoracic 

 vertebrae are made to coincide in each instance. 



The anterior half of the spinal column is considerably curved in 

 Embryo II, length 7 mm. It gradually becomes straightened in succes- 

 sively older embryos until in Embryo CLXXXIV, length 50 mm., it is 

 nearly straight. The subsequent anterior convexity in the adult is asso- 

 ciated with the assumption of an upright position of the head. 



It is, however, in the posterior half of the spinal column that the chief 

 alterations in spinal curvature are to be noted. In Embryo II, length 

 7 mm., the ventral surface of the sacral region faces the mid-thoracic 

 region; in Embryo CIX, length 11 mm., the anterior end of the vertebral 

 column; in Embr3^o CLXXXIV, length 50 mm., almost directly ven- 

 trally; and in the adult, in a posterior direction. 



The relative lengths of the various regions of the spinal column during 

 the first three months of development may be gathered from the following 

 table, which is based in part upon data obtained from embryos belonging 

 to the Mall collection and in part upon those of the Born and His collec- 

 tions studied by Aeby. 



