INTEENAL SKELETON OF VEETEBEATA. 



421 



Tlie notocliord lias always the same topographical relations to 

 the most important of the other organs. Above it, is the central 

 nervous system, and below it, the respiratory and nutrient apparatus. 

 Processes are given off from the connective tissue surrounding the 

 chord which enclose the so-called dorsal and ventral cavities ; these 

 processes pass into the musculature of the body, which is thereby 

 broken up into a number of segments, set one behind the other. 

 In Amphioxus these segments are so far asymmetrical that they 

 are found alternately on either side. 



The low condition of the axial skeleton, which is represented 

 by the chord, is permanent in the Leptocardii, where it merely 

 presents special histological modifications. In all the rest of the 

 Vertebrata the chord is the sole axial skeleton 

 in the earliest stages of development only; 

 new differentiations appear, and it becomes 

 of less physiological importance. These differ- 

 entiations affect the notochord as well as the 

 tissue which surrounds it, and which has been 

 called the "skeletogenous layer'"' or "skele- 

 togenous tissne," on account of its relations 

 to the future skeleton. The cells of the 

 chord form a tissue resembling cartilage, and 

 the sheath becomes a more independent por- 

 tion — it forms a supporting orgai; — owing to 

 the thickening of its layers. Cartilaginous 

 tissue forms around the chord (Fig. 221, b A-), 

 and that segmentation of the axial skeleton into 

 separate segments, the so-called vertebrte, 

 which was before merely indicated, now be- 

 comes obvious. This segmentation of the 

 axial skeleton is an expression of the 

 metamerism of the whole body; a series 

 of these vertebra? make up the vertebral 

 column. In each vertebra we call that por- 

 tion which surrounds the notochord the cent- 

 rum, and the outgrowing portions which enclose the dorsal and ventral 

 cavities of the body, and which are given off directly or indirectly 

 from the centrum, the arches. These again are distinguished as 

 upper or lower arches, according to their relations to these two 

 cavities. 



As the axial skeleton becomes segmented, a well-defined portion 

 in the most anterior segment forms the primitive Cranium in the 

 Craniota. 



An inferior system of arches, which encloses the most anterior 

 portion of the intestinal tract, which functions as a respiratory organ, 

 is distinguished as the branchial or visceral skeleton. _ The 

 cranium and visceral skeleton make up the most anterior portion of 

 the whole skeleton— the skeleton of the head. The other skeletal 

 structures which are connected with it are represented by the more 



Fig. 221 b. Section 

 through the spinal 

 column of a young 

 Salmon. Ch Chorda. 

 cs Chordal sheath, 

 m Spinal chord, k Supe- 

 rior, Tc' Inferior arch 

 (in rudiment), a Aorta. 

 V Yeins. 



