ON THE VERTEBRATE SKELETON. 269 



The hcemal spine is much less constant as to its existence, and is subject 

 to a much greater range of variety, when present, than is its vertical homo- 

 type above, which completes the neural arch. Long, slender, and ' spinous ' 

 in the tail, the haemal spine is reduced to a short and thick bone, often 

 flattened, in the thorax of mammals, a series of thirteen such modified spines 

 forming the so-called 'sternum' in the two-toed sloth : the thoracic haemal 

 spines are few in number, and are expanded and perforated in the whales : 

 the horizontal extension of this vertebral element is sometimes accompanied 

 by a median division, or in other words, it is ossified from two lateral centres ; 

 this is seen in the development of parts of the human sternum : the same vege- 

 tative character is constant in the broader thoracic haemal spines of birds ; 

 though, sometimes, as e. g. in the struthionidae, ossification extends from the 

 same lateral centre lengthwise, i. e. forwards and backwards, calcifying the 

 connate cartilaginous homologues of halves of four or five haemal spines, 

 before these finally coalesce with their fellows at the median line.- In some 

 other birds, however, there are two or more lateral centres, and usually, 

 also, a median one, from which the ossification of the keel extends down- 

 wards, prior to its confluence with the rest of the ' sternum.' In the thorax 

 of chelonians four haemal spines are established, each by two lateral centres 

 of ossification, forming four pairs of sternal bones with a ninth 'entosternal' 

 piece between the first and second pairs. The ' plastron ' is the result of 

 this extreme development of the haemal spines : — the modified moieties of 

 which, remaining permanently distinct and united by suture, have received 

 from Geoffroy St. Hilaire* the convenient special names of ' episternals,' 

 ' hyosternals,' ' hyposternals ' and ' xiphisternal?,' respectively, as they suc- 

 ceed each other from before backwards. 



The diverging appendages are, as might be expected, of all the elements 

 of the vertebral segment, the least constant in regard to their existence, and 

 the subjects of the greatest amount and variety of modification. Simple 

 slender spines or styles in fishes (fig. 17, a a), simple plates retaining long 

 their cartilaginous condition in crocodiles, short flat slightly curved pieces in 

 birds (fig. 15, a a), in some of the lowest species of which, e. g. Aptenodytes, 

 they become expanded, like their homologues in the crocodile ; such, with 

 one exception, is the range of the variety of form to which these parts are 

 subject in the segments of the trunk. But that exception is a remarkable 

 one : even under its normal ichthyic condition, as a simple style or filament, 

 the diverging appendage of the insulated haemapophysial portion of the pelvic 

 arch in theprotopterusf and lepidosiren £ is composed of many cartilaginous 

 segments, and projects freely from the surface, carrying with it a smooth 

 covering of integument. In other fishes similar filaments or jointed rays are 

 progressively added to the sustaining arch, which cause a progressive expan- 

 sion of the common investing fold of skin, forming the organ called the 

 ' ventral fin,' which is accordingly described by the ichthyologist as having 

 two rays (Blennius), three rays (Zoarces), up to more than twenty rays, (as 

 Acipenser in the sturgeons). 



When we quit the piscine class we find the diverging appendage of the pel- 



* Du Sternum considered dans les Oiseaux et dans les Poissons. Anatomie Philoso- 

 phique, p. 69. pi. 2, fig. 21. Here Geoffroy contends that the parts of the hyoid arch (39, 

 40 and 43) are the homologues of the modified haemal spines which he calls episternals, hyo- 

 sternals and hyposternals in the plastron of the turtle : but these names may well he retained, 

 that of ' hyosternal ' being used in an arbitrary sense, without reference to the hypothesis 

 which first suggested it. 



t Linn. Trans, vol. xviii. pi. 23, fig. 4, z. Lectures on Vertebrata, p. 79, figs. 27, 66. 



+ Bischoff, op. cit. pi. 2, fig. 5, e. 



