314 TEXT-BOOK OF EMBRYOLOGY. 



amples; for others the student is referred to the more extensive text-books of 

 anatomy. 



EXTRINSIC MUSCLES or THE UPPER EXTREMITY. The trapezius is some- 

 times attached to less than the normal number of thoracic vertebrae. Its 

 occipital attachment may be wanting. Occasionally the cervical and thoracic 

 portions are more or less separated as in some of the lower animals. 



The latissimus dorsi sometimes arises from less than the usual number of 

 thoracic vertebrae, and from less than the normal number of ribs. The iliac 

 origin may be wanting. 



The rhomboidei vary in their vertebral and scapular attachments. 



The number of the vertebral attachments of the levator scapulae may vary. 

 A small part of the muscle is sometimes attached to the occipital bone. 



The pectoralis major not infrequently varies in the extent of its attachment 

 to the ribs and sternum. 



The serrati vary in their attachment to the ribs. 



The above mentioned extrinsic muscles of the upper extremity vary prin- 

 cipally in their attachments. Since they all appear well forward in the cervical 

 region in the embryo, and, along with the extremity, gradually migrate caudally 

 before acquiring their final attachments, it is not unlikely that the variations in 

 their attachments are due to variations in the extent of migration. 



This serves to illustrate a factor which is probably important in producing 

 variations in the attachments of many other muscles. As stated in paragraph 

 i, on page 295, the myotomes frequently migrate very extensively during 

 their transformation into muscles, before the muscles have acquired their per- 

 manent attachment. Variations in the extent of this migration would naturally 

 produce variations in the final attachments of these muscles. 



Other factors related to the changes in the myotomes, such as fusion, longi- 

 tudinal and tangential splitting (paragraphs 2, 3 and 4, p. 295) probably also 

 play a part in the production of variations. 



A greater than normal degree of fusion between two or more myotomes 

 might result in the union of muscles which are usually separate; a less than 

 normal degree of fusion might result in the separation of parts usually united. 

 Variations in the splitting of myotomes might produce similar results. 



At the same time, however, heredity may be the active factor in some cases 

 where abnormal fusions or separations between muscles or parts of muscles 

 produce results resembling conditions found in lower animals. 



PRACTICAL SUGGESTIONS. 



A description of the technic for the study of the primitive segments and their differ- 

 entiation into myotomes, sclerotomes and cutis plates will be found at the end of the Chapter 

 on Connective Tissue (p. 217). The same animal and the same technic may be used for 

 the study of the succeeding stages of development of the myotomes. 



