114 OSTEOLOGY. 



posterior parts of the lungs. For the same reason the furrow on each side of 

 the spinous processes of the thoracic vertebrae is converted into a broad groove 

 (vertebral groove), the floor of which is in part formed by the ribs as far as their 

 angles. The grooves so formed are each occupied by the fleshy mass of the sacro- 

 spinalis muscle. 



The side walls are formed by the costal arches. The ribs, which run 

 obliquely from above downwards and forwards, do not lie parallel to each other, 

 but spread somewhat, so that the intervals between them (intercostal spaces) are 

 wider in front than behind. 



The superior aperture or inlet, formed by the body of the first thoracic vertebra 

 behind, the arch of the first rib on either side, and the upper border of the 

 manubrium sterni in front, is contracted and of reniform shape, measuring on an 

 average from 10 to 12 cm. transversely and 5 cm. in an antero-posterior direction. 

 The plane of the inlet is oblique from behind downwards and forwards, so that in 

 expiration the superior border of the sternum lies on a level with the fibro-cartilage 

 between the second and third thoracic vertebrae. 



The inferior aperture, of large size, is bounded in the median plane behind by the 

 twelfth thoracic vertebra ; passing thence the twelfth ribs slope laterally, downwards 

 and forwards. A line carried horizontally forwards from the tip of the twelfth rib 

 touches the end of the eleventh rib, and then curving slightly upwards reaches 

 the cartilage of the tenth rib. Thence it follows the confluent margins of the 

 cartilages of the tenth, ninth, eighth, and seventh ribs, finally reaching the xiphoid 

 process, where it forms, with the costal margin of the opposite side, the infrasternal 

 angle, the summit of which coincides with the xiphi-sternal articulation ; in 

 expiration this joint usually lies on a level with the intervertebral fibro-cartilage 

 between the ninth and tenth thoracic vertebrae, and corresponds with the surface 

 depression familiarly known as the pit of the stomach. The inferior aperture of 

 the thorax is occupied by the vault of the diaphragm. 



In the foetal condition the form of the thorax differs from that of the adult. It is 

 compressed from side to side in this respect resembling the simian type. Its antero- 

 posterior diameter is relatively greater than in the adult. At birth, changes in form take 

 place dependent on the expansion of the lungs ; during subsequent growth, the further 

 expansion of the thoracic cavity in a transverse direction is correlated with the assumption 

 of the erect posture, and the use of the fore-limbs as prehensile organs. 



Sexual Differences. The thorax of the female is usually described as being pro- 

 portionately shorter and rounder than the male. It also tends to narrowness in the 

 lower segment. It is hardly necessary to point out that the natural form is often 

 modified by the use of tight or ill-fitting corsets. 



mi TM- T j Transverse diameter x 100 , > ,, , -, . ,, . ,. 



The Thoracic Index = taken at the level of the junction 



Antero-posterior diameter 



of the xiphoid process with the body expresses the proportions of these diameters. That 

 of the female is on an average lower than the male, indicating a more rounded form. 



THE SKULL. 



(In view of the vast amount of accurate knowledge the medical student is now called upon to 

 acquire, it is, in the opinion of the writer of this article, desirable that less stress should be laid 

 upon the details of the disarticulated bones of the skull and more emphasis placed on the study 

 of the skull as a whole. 



It has hitherto been the custom to disarticulate the bones of the skull, imposing on the 

 student the task of again reconstructing it, much after the manner of a Chinese puzzle. In this 

 way a minute acquaintance with, the forms and articulations of the individual bones became 

 necessary, and the student's memory was burdened with a mass of detail of little or no practical 

 or scientific value, for in regard to the latter aspect of the subject the points of phylogenetic and 

 ontogenetic interest are best illustrated by a consideration of the details of the evolution of the 

 skull and the development and ossification of its parts. With possibly the exception of the 

 temporal bones and the mandible, the author holds that most of the useful information relating 

 to the skull can best be studied in the complete cranium, or in sections of it made in different 

 planes. By this method the student acquires a more intimate knowledge of its structure and 

 topography, and is consequently better equipped to deal with the regions he may have to explore 

 in the living. 



