VAKIATIONS IN THE SKELETON. 275 



arrangement of fibres is evident, and the plantar surface of the neck of the talus is further 

 strengthened by lamellae arranged vertically. 



In the separate bones the investing envelope is thin, though under the articular surfaces 

 there is a greater density, due to the accession of lamellae lying parallel to the articular planes. 

 The stoutest bony tissue in the talus is met with in the region of the plantar surface of the 

 neck, whilst in the calcaneus the greatest density occurs along the floor of the sinus tarsi. 



The Metatarsus. In structure and the arrangement of their lamellae the metatarsal bones 

 agree with the metacarpus. 



The Phalanges. In their general structure they resemble the bones of the fingers. 



APPENDIX B. 



VARIATIONS IN THE SKELETON. 



Cervical Vertebrae. Szawlowski records the presence of an independent rib element in the 

 transverse process of the fourth cervical Vertebra. (Anat. Anz. Jena, vol. xx. p. 306.) 



Atlas. The foramen transversarium is often deficient in front. Imperfect ossification occa- 

 sionally leads to the anterior arch, and more frequently the posterior arch, being incomplete. The 

 superior articular surfaces are occasionally partially or completely divided into anterior and 

 posterior portions. In some instances the extremity of the transverse process has two tubercles. 

 The transverse process may, in. rare cases, articulate with a projecting process (paroccipital or 

 paramastoid) from the under surface of the jugular process of the occipital bone (see p. 278). An 

 upward extension from the medial part of the anterior arch, due probably to an ossification of 

 the anterior occipito-atlantal ligament, may articulate with the anterior surface of the summit of 

 the dens of the epistropheus. Allen has noticed the articulation of the superior border of the 

 posterior arch with the posterior border of the foramen magnum. Cases of partial or complete 

 fusion of the atlas with the occipital bone are not uncommon (see p. 278). 



Epistropheus. In some instances the summit of the dens articulates with a prominent 

 tubercle on the anterior border of the foramen magnum (third occipital condyle, see p. 278). 

 Bennett (Trans. Path. Soc. Dublin, vol. vii.) records a case in which the dens was double, 

 due to the persistence of the primitive condition in which it is developed from two centres, 

 Occasionally the dens fails to be united with the body of the epistropheus, forming an os 

 odontoideum comparable to that met with in the crocodilia. (Giacomini, Romiti, and Turner.) 

 The foramen transversarium is not infrequently incomplete, owing to the imperfect ossification 

 of the posterior root of the transverse process. Elliot Smith has recorded a case in which there 

 was fusion between the atlas and epistropheus without any evidence of disease. 



Seventh Cervical Vertebra. The foramen transversarium may be absent on one or other 



side. 



Thoracic Vertebrae. Barclay Smith (Journ. Anat. and Physiol. Lond. 1902, p. 372) records 

 five cases in which the superior articular processes of the twelfth thoracic vertebra displayed 

 thoracic and lumbar characteristics on opposite sides. Duckworth (Journ. of Anat. and Physiol. 

 vol. xlv. p. 65) has described a first thoracic vertebra, in which a bony process, arising from the 

 front of the root of the transverse process, curves forwards and medially so as almost to enclose a 

 foramen like that of the cervical vertebrae. The ventral surface of this process articulates with 

 the neck of the first rib. 



Lumbar Vertebrae. The mamillary and accessory processes are sometimes unduly de- 

 veloped. The vertebral arch of the fifth lumbar vertebra is occasionally interrupted on either 

 side by a synchondrosis which runs between the upper and lower articular processes. In 

 macerated specimens the two parts of the bone are thus separate and independent. The 

 anterior includes the body, together with the roots of the vertebral arches and. the transverse 

 and superior articular processes ; the posterior comprises the inferior articular processes, the 

 laminae, and the spine. (Turner, Challenger Reports, vol. xvi.) Fawcett has seen the same con- 

 dition in the fourth lumbar vertebra. Szawlowski and Dwight record instances of the occurrence 

 of a foramen in the transverse process of the fifth lumbar vertebra (Anat. Anb. Jena, vol. xx.), 

 and Ramsay Smith describes a case in which the right transverse process of the fourth lumbar 

 vertebra of an Australian sprang from the side of the body in front of the root of the vertebral 

 arch, being unconnected either with the arch or articular process. 



Sacrum. The number of sacral segments may be increased to six or reduced to four (see 

 p. 276). Transition forms are occasionally met with in which the first sacral segment displays 

 on one side purely sacral characters, i.e. it articulates with the hip bone, whilst on the 

 opposite side it may present all the features of a lumbar vertebra. Through deficiency in the 

 development of the laminae, the sacral canal may be exposed throughout its entire length, or 

 to a greater extent than is normally the case. (Paterson, Roy. Dublin Soc. Scientific Trans. 

 vol. v. Series II.) Szawlowski and Barclay Smith record the occurrence of a foramen in the 

 lateral part of the first sacral vertebra. (Journ. of Anat. and Physiol. Lond. voL xxxvi. p. 372.) 



Vertebral Column as a Whole. Increase in the number of vertebral segments is usually 



