222 



ANATOMY. 



frequently lap over one nnolher, to diminish tin- 

 size, during the passage ut thu heud through tin- 

 pelvis. 



The fibres of the bones of the fo_>tal cranium are 

 seen diverging from a central point of ossification to- 

 wards the circumference, and each bone appears to 

 be composed of a single lamina, so that ossification 

 is going on from a considerable number of points 

 at the same time, and the bones consequently ap- 

 proach each other in the same proportion till they 

 finally arrive at the state of perfect bone, of which 

 a description has been already given. 



Having now completed all we intended on the 

 osteological description of the skull, or cranium, 

 and face, we shall next consider, and that very 

 briefly, the bones of the trunk, and then those of 

 the extremities. The relative shapes, connections, 

 and sizes of these bones are more distinctly seen in 

 the anterior and posterior views of the skeleton 

 than those of the head, which is the most impor- 

 tant part in an osteological bust, in every other 

 point of view, and therefore, a more full and ex- 

 tended demonstration of its structure than is given 

 of any of the other divisions of the bony system 

 was more especially called for. 



TRUNK. The bones of the trunk are composed 

 of the spine or back bone, the thorax or chest, and 

 the pelvis or hips. The spine, although spoken 

 of as one bone, consists of not less than twenty- 

 four bones, called vertebrae, which are arranged 

 into three classes, the cervical, or those of the 

 neck, (7) the dorsal, or those of the back, (12) 

 and the lumbar, or those of the loins, (5.) As a 

 short anatomical description of these different ver- 

 tebrae, and of the intervening compressible and 

 cork-like intervertebral substance will be found 

 under Verttbra, in the Encyclopedia, we refer to 

 that article, and only notice, in this place, some of 

 those circumstances which have either been omit- 

 ted, or only very shortly noticed. 



It will be seen by consulting the article referred 

 to, that the spine is a bony column, the volume of 

 the vertebrae being very considerable in the lum- 

 bar region, but decreasing in size as they ascend to 

 the head, though with some irregularity, and form- 

 ing a true natural pyramid, reposing its base on the 

 sacrum, or large posterior bone of the pelvis, as a 

 sort of pedestal, and having on its summit the head. 

 Indeed, when taken altogether, it is an admirable 

 structure, and it is no matter of surprise to us 

 that almost every writer on natural theology should 

 have selected this part of the human body as a 

 specimen of the most exquisite mechanism. 



The bodies of the vertebrae are, as already ob- 

 served, connected by the intervertebral substance, 

 and posteriorly by a yellow elastic ligament and 

 1/y their oblique processes. The intervertebral 

 substance, the peculiar nature of which has 

 been hinted at in the foetus, resembles more the 

 structure of ligament, and in the adult it has a 

 greater similitude to cartilage. Externally it is 

 firm and hard, internally it becomes thinner and 

 softer, and in the centre* it is like mucus. A 

 considerable change takes place in this substance 

 in advanced life, when it becomes shrivelled, and 

 loses in a great measure its elasticity, which occa- 

 sions the decrease in stature, and the stooping for- 

 wards. Besides this change in old age, this sub- 

 stance also undergoes a temporary diminution after 

 the fujl growth of the suoject, from the weight of 

 the body in an erect posture, so that people who 

 bave been long standing, or have carried a consid- 



erable weight, iire found to be shorter, than when 

 they huve been long in bed; hence, we are talh r 

 in the morning than at night. By the aid of this 

 intervertebral substance, and the processes and 

 articular cavities of the bones of the spine, we 

 have the finest specimens of motion by ginglyattit, 

 as shown in the connection of the atlas with 

 the occipital bone of the trochoides, or that species 

 of joint, or articulation, in which one bone moves 

 round upon another, as in the connection of tin- 

 first and second cervical vertebras; and of syndes- 

 mosis, or that kind of movement so freely permit- 

 ted by the union of bones by ligament, all which 

 examples may be seen in the space of little more 

 than one inch, in the connection of the head and 

 spine, forming by far the most important articula- 

 tion of the body. This, however, is not all ; lor 

 the trochoid joint formed by the anterior arch of 

 the atlas moving round the odontoid process of the 

 dentatus, permits the head not only to rotate, as in 

 moving from side to side, but as the head cannot, 

 move sufficiently round the dentatus without in- 

 juring the spinal marrow, all the cervical vertebrre 

 likewise concur in the rotatory motion when in any 

 considerable degree. Strong ligamentous fibres, too, 

 pass from the sides of the odontoid process, over 

 the atlas, to the os occipitis ; which not only 

 strengthen the articulation, but also limit its mo- 

 tions ; and this odontoid or tooth-like process, is far- 

 ther retained in its situation by the transverse liga- 

 ment of the atlas. 



To those facts we may add, that numerous mus- 

 cles are attached to the back part of the spine, and 

 several on the forepart. 



This beautiful structure, the spinal column, is, 

 however, like every other part of our frame, liable 

 to accidents and disease ; but we shall only notice 

 those most common to the vertebrae. When an 

 aneurism of the aorta is very large, the bodies of 

 the vertebras are occasionally absorbed, but the in- 

 tervertebral substance remains entire. The writer of 

 this article met with a case of aneurism in the aorta 

 about twenty years ago, in a man of about forty-five 

 years of age, in which a post mortem examination was 

 permitted, and four of the ribs were found to be 

 partially absorbed, and part of the bodies of two 

 of the dorsal vertebrae. The attentive reader will 

 not fail to notice the extraordinary fact, that bone 

 should be sooner absorbed than cartilage, a much 

 softer and yielding substance ; but the same pheno- 

 menon takes place in caries from other causes, and 

 the spinal marrow is always defended in these cases 

 by a deposit of coagulable lymph. Caries, distor- 

 tion, and hydrorachitis, or spina bifida, are also not 

 unfrequent affections of the spine. This latter dis- 

 ease, viz. spina bifida, consists of a tumour filled 

 with water, which makes its way between the pro- 

 cesses of a vertebra, and divides it in two. 



In conclusion, much of the animal mechanism of 

 the spine may be learned from examining the spine 

 of some of the lower animals ; and it may not be 

 altogether useless to learn how to know any sepa- 

 rate vertebra of the spine that may be found, and 

 the class to which it belongs, which may be easily 

 done by recollecting, if our descriptions are borne 

 in mind, that the peculiar characteristic of a cer- 

 vical vertebra is the perforations in the trans- 

 verse processes. The dorsal vertebrae are distin- 

 guished by having articular surfaces, for the heads 

 of the ribs, and the lumbar by their size and the 

 length of their transverse processes. 



The Thorax or Chest, is formed by the Costa or 



