1006 



OF GENERATION. 



the bodies of the vertebras, which are still hollow, so as to allow the segments 

 of the chorda, partially separated from each other, to communicate together. 

 This condition also remains persistent in certain of the Cartilaginous Fishes. 

 With the concentric growth of the bodies of the vertebrae, however, the chorda 

 dorsalis gradually wastes, and at last disappears; but previously to its disap- 

 pearance, the ossification of the bodies and neural arches of the vertebrae 

 begins, the former from a single point on the median line, the latter by separate 

 points on the two sides. The complete typical vertebra (Fig. 274, A) essentially 

 consists, according to Prof. Owen, 1 of the centrum, around which are arranged 

 four arches inclosed by processes in connection with it : viz., superiorly, the 

 neural arch, which incloses the neural axis, and is formed by a pair of "neura- 

 pophyses" (fc, n) and a "neural spine" (n, s) ; inferiorly, the hgemal arch, which 

 is in like special relation with the centres of the circulation, but may be ex- 

 panded around the visceral cavity generally, and which is formed of a pair of 

 " hsemapophyses" (h, h), and the " hsemal spine" (h, s) ; and two lateral arches, 

 inclosing vascular canals, which are bounded by the " diapophyses" (d, d) 

 and the " parapophyses" (p, p), and are completed by the " pleurapophyses" 

 (pi, pi). Of these elements, the centrum is the most constant ; and next^to 

 these are the neural arches, which we find in every part of the vertebral 

 column through which the neural axis passes, and which are enormously de- 

 veloped in the cranial segments, in accordance with the high development of the 

 nervous mass. The haemal arches are often almost entirely deficient, as in the 



Fig. 274. 



IB ns 



Elements of a Vertebra, according to Prof. Owen : A, ideal typical vertebra ; B, actual thoracic vertebra of a 

 Bird ; c, centrum, giving off d, d, the diapophyses, and p, p, the parapophyses ; the neural arch, inclosing 

 the spinal cord *, is formed by n, n, the neurapophyses and n, s, the neural spine; the haemal arch, inclosing 

 the great centres of the circulation, is formed by h, h, the haemapophyses, and h, s, the haemal spine. From 

 both the neurapophyses and hasmapophyses may be given off the zygapophyses, z, z. The lateral arches, 

 which may inclose the vertebral arteries o, are completed by the pleurapophyses, p, 1; these in B are bent 

 downwards, so as to form part of the haemal arch, and give off the diverging appendages a, a. 



cervical and lumbar vertebras of Man and the Mammalia : but in the dorsal verte- 

 brae they are very largely developed, and the elements of the lateral arches are 

 brought into connection with them, so as to form the inclosure of the visceral 

 cavity (Fig. 274, B). From the pleurapophyses are occasionally developed a pair 



1 See his "Archetype Skeleton," his "Lectures on Comparative Anatomy," TO!, ii., and 

 his "Discourse on the Nature of Limbs." 



