330 Vertebraf.a. 



another, and closely vmited to tliese at their ends ; the limb muscles 

 are usually also very powerful. They consist of striated fibres, bound 

 together by connective tissue. Usually they terminate in tendons, 

 which consist of fibrous connective tissue ; the tendons are not 

 infrequently, especially in Mammalia and A.ves, of considerable 

 length. Sometimes they may be ossified to a greater or less extent, 

 and often small "sesamoid bones" develop in those portions 

 which pass over a bone, having cartilaginous surfaces towards 

 the bone; the knee-cap (patella) of Birds and Mammals is a 

 sesamoid bone. 



The central nervous system arises in the Vertebrata 

 along the dorsal surface of the animal, as a grooved infolding 

 of the epiblast (Pig. 39, p. 48), which is later cut off from the rest of 

 the layer, and lies as a tube below the skin. In Amphioxus it remains 

 in this state throughout life ; in others it is modified anteriorly, 

 to form a b r a i n, in contradistinction to the rest, the spinal cord. 

 The lumen of the cord usually persists as a narrow canal (the 

 central canal) in the spinal cord, in the form of larger cavities (the 

 ventricles) in the brain. The brain is, from very early stages, divided 

 by grooves into three regions, of which the first and last are again 

 sub-divided into two. There are thus five sections : primary 

 and secondary fore - brain (th alamencep halon, prosen- 



Fig. 272. Diagrammatio vertical longitudinal section through a vertebrate brain. 

 / cerebrnm, me thalamencephalon, 7ni mid-brain, b cerebellum, e medulla, I olfactory lobes, 

 k epiphysis, ir hypophysis, t pitaitary body. — Orig. 



cephalon); mid-brain (mesencephalon) ; primary and 

 secondary hind-brain (metencephalon, myelencephalon); 

 they lie one behind the other, and can be distinguished throughout 

 the whole series of Vertebrata, from Pisces upwards, although in 

 other respects the structure of the brain, as a whole, and of its 

 various parts, exhibits great variety. The most anterior section, 

 the prosencephalon, or cerebrum, which is usually well deve- 

 loped, and, especially so in higher Vertebrata (Birds and Mammals), 

 is generally divided into two halves (the cerebral hemispheres) by 

 a longitudinal fold which dips into it from above and before, and 

 these hemispheres are prolonged anteriorly into a pair of small 

 hollow bodies, the olfactory lobes; the wall of the cerebrum 

 is much thickened both above and below. The thai am- 



