Feb 



1883.] 



KNOWTLEDGE 



67 



"OUR BODIES:" 



SHORT PAPERS ON PHYSIOLOGY. 



By Dr. Andrew Wilson, F.R.S.E., ic. 



Xo. v.— SKULL AND SPINE. 



ONE very perceptible difference between the lower and 

 " back-boneless," or Invertchraled animals, and the 

 " back-boned," or Vi:Ttilrat€d series, consists in the posses- 

 sion by the latter of hard internal parts, named the skeleton. 

 Perhaps it would be more correct to say the endo- skeleton, 

 or inner skeleton, for the plain reason that the backboned 

 animals may, and often do, possess a very well-defined 

 outer skeleton as well. We, ourselves, possess an outer 

 skeleton in the form of our hairs, nails, and teeth. All of 

 these parts are formed from and by the skin-layers, and 

 possess no relationship with the bones forming the inner- 

 skeleton. From the fishes to mammals (or quadrupeds), 

 including man, an internal skeleton is present in some 

 form or other. It may appear simply as a soft cellular 

 rod, the notochord, lying along the back of the animal, 

 and showing no apparent affinity to the skeleton as we 

 know that structure in higher life. Such is the skeleton 

 in the lowest fish, known as the Laneelet. Then, as in sharks, 

 rays, ic, the skeleton may remain permanently in a gristly 

 (or cartilaginous) condition ; whilst finally, hx most fishes 

 (cod, salmon, kc), it is composed of hone. We seem to 

 see that the older fishes, or those which are first known in a 

 fossil state, such as the sharks and ganoids, represent, in 

 so far as their skeletons are concerned, an early condi- 

 tion of the modern and bony fishes. Bone, as we learned 

 in our first paper, grows from cartilage or gristle ; and it 

 would thus seem as though in the growth of a single 

 bone we see a mo\ing picture of the development of 

 skeletons in past time. 



The " backboned " animals, or Vertebrates, as most 

 readers know, number five classes. These are, firstly, 

 fishes; secondly, amphibians (frogs, toads, newts, ic), 

 often confused with, thirdly, the reptiles ; fourthly, birds ; 

 and fifthly, mammals. Man, I need hardly say, takes his 

 zoological position at the head of the last-named class. 

 Leaving out the lowest fishes, and some of the older types 

 as well, we find that all vertebrates possess an internal 

 bony framework, the skeleton, which, besides aflbrding 

 support for the soft parts, and for the attachment of 

 muscles, also forms cavities {e.g., the skull and chest) for 

 the protection of important organs. It is true that, even 

 in man's own class — not to speak of the fishes, reptiles, 

 and birds — there are animals which may possess a well- 

 developed outside skeleton. The armadillos of South 

 America, the pangolins of Asia and Africa, the hedgehogs 

 and porcupines, all present us with an outside skeleton of 

 more or less complete character. But, in addition, all the 

 backboned animals, from fish to man, evince a common 

 type in the structure and build of their bony parts. We 

 can discern in all, the spine, vertebral column, or backbone, 

 which forms the main axis of the body, and which expands 

 in front or above to form the skull. Then, secondly, we 

 see the limbs and their skeleton attached to this main axis. 

 So that it would be perfectly correct in the first instance 

 to say that all vertebrated animals possess a skeleton which 

 is divisible, firstly, into the trunk or axis, and, secondly, 

 the appendages or limbs ; and anatomists have accordingly 

 come to speak of the a.cial skeleton and the appendicular 

 skeleton, as representing a natural division of the bony 

 belongings of the body. 



The spine or backbone forms a very convenient starting- 

 point for a study of the skeleton. It consists in man of 



thirty-three bones, each called a vertebra. Each vertebra 

 is built on a plan similar to that seen in its neighbour- 

 bones. One common plan thus runs throughout the ele- 

 ments of the spine. Each of the vertebne consists of a solid 

 part, called its ciitruin or bodg ; and it is the series of 

 " bodies," which, when arranged in due order, give solidity 

 to tlie spine. From the body arise certain projections 

 called processes. Above, there is an arch which ends 

 in a process called the spinous process. It is the series 

 of these latter processes we feel when we stroke a cat's 

 back, or when we feel our own spine, as in the back of 

 the neck, for example. Other (transrcrse) processes — for 

 the attachment of muscles like the spinous processes — 

 are given off at the sides of the vertebr.-B ; and lastly there 

 are projections (arlirulur j/rocesses), by means of which each 

 vertebra interlocks with its neighbours. But a very im- 

 portant observation is that which shows us that in reality 

 the spine is a hollow tube. Each vertebra has an arch, as 

 we have seen ; that is, part of it forms a bridge. If we 

 join a number of bridges together, we form a tunnel. In 

 our bodies, then, the vertebr.T, united end to end, form a 

 tunnel, or continuous tube, within which the sjnnal cord or 

 spinal inarroin — the great main nerve-trunk of the body — 

 lies. The hollow of the .spine, moreover, is continued upwards 

 into the inside of the skull ; the brain and spinal marrow 

 thus becoming continuous. If we look at the section of a 

 sheep or ox, whose body the butcher has cleft in two 

 lengthwise down the middle of the spine, we may see how a 

 tube is formed by the spine, and how, within this tube, the 

 spinal cord is lodged. In truth, the body of all vertebrate 

 animals, from the fish to the man, consists of two parallel 

 tubes. One tube is formed by the spine and skull, as we 

 have seen, and contains the nervous system. The other 

 tube is formed by the general walls of the body, ribs, etc., 

 and contains the other organs (heart, lungs, digestive 

 organs, etc.), of our frames. 



In the spine of man there are in all thirty-three bones. 

 If we tabulate them as follows, we may gain a clear idea of 

 the composition of the spine : — 



(1) Neck Vertebrae 7 in number. 



(2) Back „ 12 



(3) Lumbar or Loins Vertebrae 5 ,, 



(4) Sacrum 5 ,, 



(5) Coccyx (or tail) 4 ,, 



Total 33 



The first three regions, neck, back, and loins, are readily 

 recognised. The sacrum is a part of the spine, consisting 

 of a single bone, formed of five united vertebrii', and which 

 is wedged in between the haunch-bones ; whilst the coccyx 

 is the rudimentary tail, composed of four small and de- 

 graded vertebra'. Man's spine is very short as compared 

 with that of many other animals. A serpent, for example, 

 has an extremely long backbone, consisting in some cases 

 of over 400 vertebr;e ; and a spider-monkey may have 

 more bones in his tail than man possesses in his entire 

 spine. Man's spine exhibits a series of very beautiful 

 curves. Its average length in the adult is about 28 inches, 

 and its curves adapt it for the efTective support of the 

 weight of head and body. Certain curves are found in 

 humanity alone. For example, it is only in the human 

 spine that we find the vertebra? of the loins curved or 

 convex forward. At birth, the spine is well-nigh straight, 

 save for a slight concavity or hollowing forward in the 

 back. But when the erect position begins to be assumed, 

 as when the child is held upwards in the sitting posture, 

 the neck-region begins to grow convex forwards, and the 

 curving forwards in the loins also appears. 



The skull was formerly regarded as merely a mass of 



