106 MERISTIC VARIATION. [part i. 



cartilage or by two cartilages. The cartilage of the left 7th rib is 

 also double for H inch, all the others are normal. 2. Left fourth 

 rib becoming very broad and bifurcating in front ; two large spaces, 

 one in the bone, one at the bifurcation. 3. Left fourth rib becoming 

 broad towards sternal end, where it joins bifurcated cartilage. In 

 these three cases the division affected the 4th rib. Three others 

 are given in which the rib affected was probably the 4th or 5th. 

 Struthers, J., Jour. Anat. Phys., Ser. 2, VIII. 1875, p. 51. Such 

 cases are often recorded and preparations illustrating them may 

 be seen in most museums. 



Besides these cases of obviously Meristic Variation, there are 

 many which are combined with Homceosis so as to produce far 

 greater anatomical divergence. Though in some of these examples 

 there may be change in the total number of vertebra? shewing 

 that true Meristic change has occurred, they cannot well be treated 

 apart from the more distinctly Homceotic cases. 



Homceotic Variation in Vertebrae and Ribs. 



Homceosis in vertebrae may be best studied in Mammals, 

 and the following account in the first instance relates chiefly to 

 them. Before considering the details of such variations in vertebrae, 

 it may be useful to describe briefly the ordinary system of nomen- 

 clature which is here followed. In treating this subject it is im- 

 possible to employ a terminology which does not seem to imply 

 acceptance of the view that there is a true homology between the 

 individual vertebrae of two spines containing different total num- 

 bers, for all the nomenclature of Comparative Anatomy is devised 

 on this hypothesis. This difficulty is especially felt in regard to 

 vertebrae, and at this point it should be expressly stated that in 

 using the ordinary terms no such assent is intended. This matter 

 has already been referred to in Section VI. of the Introduction, 

 and will be discussed in relation to the facts to be given. 



The vertebral column 1 is divided into five regions: — cervical, dorsal, lumbar, 

 sacral and caudal. None of these regions can be absolutely defined, but the following 

 features are generally used to differentiate them. 



Cervical vertebrae are those of the anterior portion of the column, which either 

 have no moveable ribs, or else have ribs which do not reach the sternum. Dorsal 

 vertebrae are those which lie posterior to the cervicals and have moveable ribs. 

 Lumbar vertebras are those which succeed to the dorsals and have no moveable ribs. 

 Sacral vertebrae cannot be defined in terms applicable even to the whole class of 

 mammals, but, for the purpose of this consideration, it will be enough to use the 

 term in the sense ordinarily given to it in human anatomy, to mean those vertebra? 

 which are ankylosed together to form a sacrum. Caudal vertebrae are vertebra? 

 posterior to the sacrum. 



The characters thus defined are distributed among the several 

 vertebrae according to their ordinal positions. Among mammals 

 the number of vertebrae which develop the characters of each re- 



1 Abridged from Flower, W. H., Mammals, Living and Extinct, 1891, p. 41. 



