PHYLUM CHORDATA 



95 



consists of the drainage from the tissues : it makes its way into 

 the lymph capillaries, and thence into the lymphatics, which are 

 all efferent vessels, conveying the fluid from the capillaries to the 

 veins. Leucocytes are added to the plasma in bodies called 

 lymphatic glands, which occur in the course of the vessels. Valves 

 may be present to prevent any flow of lymph towards the 

 capillaries, and in some cases the flow of the fluid is assisted by 

 lymph-hearts, muscular dilatations in the course of certain of the 

 vessels. The lymphatics of the intestine have an important 

 function in the absorption of fats, and are known as lacteals (lc.). 



The nervous 

 system attains a 

 complexity, both 

 anatomical and his- 

 tological, unknown 

 in the rest of the 

 animal kingdom. It 

 arises, as in other 

 Chordata, from a 

 dorsal medullary 

 groove the edges of 

 which unite and en- 

 close a tube. From 

 the ectoderm lining 

 the tube the whole 

 central nervous 

 system, or neuron, 

 is formed ; its lumen 

 forms the neuroccele 

 or characteristic 

 axial cavity of the 

 neuron. So far the 

 agreement with the 

 lower Chordata is 

 complete, but a fun- 

 damental advance is 

 seen in the fact that 

 at an early period 

 before the closure of the medullary groove the anterior end ol 

 the neuron undergoes a marked dilatation and forms the rudiment 

 of the brain, the rest becoming the spinal cord. Moreover, as growth 

 goes on a space appears in the mesoderm immediately surrounding 

 the nervous system, and forms the neural or cerebro-spinal cawty 

 already referred to (Fig. 769, C.S.G.), so that the neuron, instead of 

 being solidly imbedded in mesoderm, lies in a well-marked and 

 often spacious tube enclosed by the neural arches of the vertebrae, 

 and in front by the cranium (Fig. 769, B-D). 



786 Transverse section of spinal cord. 1, ventral 

 fissure ; 2, dorsal fissure ; 3, central canal -4, 5, badges 

 connecting grey matter of right and left sides ; 6, 7, I. 

 white matter ; 9, dorsal root of spinal nerve ; 10 ventral 

 root a b dorsal horn of grey matter ; c. Clarke s column ; 

 e ventral horn. (From Huxley's Physiology.) 



