484 



SKELETON 



li rmiirs- of cartilage. is called the notochord, and is 

 one of the constant characteristics of the skeleton of 

 Vertebrates. There U a hint of it in the proboscis 

 of Balanoglossus (t|.v.) anil in Cephalodiscus (q.v.) ; 

 it is more distinct in the tail of ytmnx Aseidians 

 (q.v.), and persist* throughout life in Appendiq- 

 ufaria : in the lancelet, in the hag, anil in the 

 young lamprey it is an unsegmented rod with a 



simple sheath ; 

 in the adult 

 lamprey there 

 are above the 

 notochord 

 rudimentary 

 arches of carti- 

 lage forming 

 a trough in 

 which the 

 spinal cord 

 lies ; in cartila- 

 ginous Ganoid 

 fishes, such as 

 the sturgeon, 



Fig. 1. in the Chimera 



A, noMcmented notochord (d), IU sheath type, and in the 

 (). B. notochord in process of being iy nno : parti 

 constructed by it. sheath, the part. W 

 marked (v corresponding to th centred laginousarclies 

 of two adjacent vertebra. C, Mctlon or appear both 

 a young vertebral column: d, the noto- jalmva ami lw> 

 chord;*, ita .heath; b, the spinal cord f ,, " 

 protected by neural arche. (o); beneath "** tlle noto- 

 are Inferior processes (). chord, but 



there are as 



yet no vertebral bodies. These begin in the 

 Elasmobranch fishes, in which the notochord is, 

 in part at least, constricted bv the encroach- 

 ment of its sheath, and divided into vertebne. 

 In the bony Ganoids, such as Lepidosteus, the 

 vertebra are ossified, and so they are more or 

 less thoroughly in all the higher Vertebrates. 

 Yet it is not the notochord which is ossified, 

 but its sheath ; the notochord in all higher Ver- 

 tebrates being merely a provisional structure, 

 an internal scaffolding around which its 'substi- 

 tute,' the backbone, is built Each vertebra 

 generally consists of the substantial body or 

 centrum, the neural arches which form a tube for 

 the spinal cord and meet altove it in a neural 

 spine, the transverse processes which project later- 

 ally and are usually connected with ribs, and the 

 articular processes which hind vertebra to vertebra 

 so that a linn and yet flexible backbone results. 



A breastbone or sternum to which the ribs are 

 ventrally united occurs in many Reptiles and jn all 

 Birds and Mammals. It arises from a cartilaginous 

 tract uniting the ventral ends of the ribs. In 

 Amphibians also there is a breastbone, but there 

 are no distinct ribs, and it is not certain that we 

 can compare this sternum with that of higher 

 Vertebrates. 



The Skull. In a young Vertebrate animal the 

 cavity in which the urain lies is surrounded by a 

 membranous sheath, but this is gradually replaced, 

 lii-t l>y a gristly brain-box, and afterwards in most 

 cases' by an almost entirely bony skull. Let us first 



tin! gristly brain-box or chondro-cranium. 

 ('O Its foundation is formed from two pairs of 

 cartilaginous plates posterior parachordals and 

 anterior trabeculie which lie beside and in front 

 of the notochord. These are extended upwards by 

 a further formation of cartilage ; the end of the 

 notochonl may also help a little : the result is a 

 cartilaginous brain-box. (6) But to the sides of 

 this are added a pair of cartilaginous nasal capsules 

 in front, and a similar pair of auditory capsules 

 Ix'liiiul. (r) About the mouth there are some lip 

 or labial cartilages, which may help in forming the 

 skull ; but much morn important is a series of 

 cartilaginous ' branchial arches ' ( never more than 



eight pairs), which loop round the pharynx, run 

 niug between the primitive gill clefts, (if these 

 arches the two most anterior, which are calleil the 

 mandibnlar and the hyoid arches, are of great im- 

 |irt:ince in the development of the skull ; the 

 others form sn]>i>orts for the pharynx, and are per- 

 manently im|Kiiiant only in Fishes and in gUled 

 Amphibians. In ElasmoWaiich fishes the mandib- 

 ular and hyoid arches do not form any direct part 

 of the gristly 

 brain-lmx, but 

 in the Teleo- 

 steans and 

 thence on- 

 wards they, 

 or the bones 

 which replace 

 them, contrih- 

 ute directly to 

 the upbuild- 

 ing of the 

 skull. To fol- 

 low the history 

 of t he arches, 

 which under- 

 go numerous 

 transforma- 

 tions, is one 

 of the most 

 difficult and 



Fig. 2. 



A, cartilaginous rudiment, of the skull: 

 parachonials ;;, trabecute /, anterior 

 end of notochord (d . auditory capsule , 

 optic capsule ( 6), nasal capsule i a . B, a 

 slightly more advanced state: the nanal 

 and auditory capsules have fused with the 

 brain-box; the optic capsules remain free. 



interesting tasks of comparative anatomy, (d) 

 When a i>one develops in direct relation to a 

 prc-cxistent cartilage which it replaces, it is often 

 called a primary or 'cartilage bone;' and then- 

 are many regions of the cartilaginous brain- 

 IK>X which in the course of development are thus 

 replaced by bones. But there are other bones which 

 develop independently of pre-existent cartilage. 

 They invest tlie cartilaginous brain-lxix on its roof, 

 on its floor, and on its sides. They are comparable 

 to the dermal ossifications or scutes which occur in 

 Ganoid fishes and many other animals, and they 

 are often called secondary or ' membrane ' bones. 

 In structure they are of course indistinguishable 

 from ' cartilage bones ; ' in origin too they are in 

 one way the same, for all bones arise 'from a 

 (periostea!) membrane of bone-making cells; but 

 'cartilage bones' are at one time represent eil by 

 cartilages, whereas ' membrane bones ' never arc. 



To sum up, the skull is formed (a) from the para- 

 chonials and t ral iffiil:f at the end of the notochord, 

 (b) from the adjacent sense-capsules of the nose 



A i, 



Fig. 3. Diagram showing the branchial arches of the 



Embryo : 



d and p, parts of first or mandlbular arch ; /, part of second or 

 hyoid arch ; behind these the arches ( A ) separating gill-clefts ; 

 o, b, r, the nasal, optic, and auditory capsules. 



and the ear, (<) from the more or less intimately 

 associated mandihular and hvoid arches, (d) from 

 the ossification of the gristly brain-lmx due to (a), 

 (6), and (c), but also from 'membrane bones ' 

 or investing bones which arise independently of 

 pre-exUtent cartilages. There is no skull in 

 Tnnicates or in the lancelet ; it is cartilaginous 

 in Cyclostomata and Elasmobranchs ; centres of 



