326 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



filiation of the oxygen and the haemoglobin is a very loose one and is 



readily broken up. This chemical peculiarity of haemoglobin adapts it 



admirably to the function which it performs in the body, that of serving 



as a vehicle for the conveyance of oxygen throughout the living tissues. 



ing through the capillary network of the respiratory organ the 



haemoglobin becomes oxidized by oxygen from the external medium and 



the blood as a consequence takes on the bright scarlet characteristic of 



rial blood. Each corpuscle whirled onwards in the blood-stream 



retains its complement of oxygen until at last it is brought, in the 



capillary network, into the immediate proximity of tissue hungry for 



there the combination is dissolved, the oxygen is retained by 



the tissue, while the haemoglobin is borne away in the now dark-coloured 



venous blood, eventually to reach again the respiratory surface where 



the cycle is started afresh. 



The blood serves also for the transport of other substances, such as 

 I'ood materials, and carbon dioxide and other excretory products, but in 

 their case it is not known what special mechanisms, if any, exist for the 

 purpose. 



The walls of the blood-vessels are not absolutely impermeable. 

 Plasma oozes out and leucocytes wander out by their own activity. The 

 colourless blood so constituted is known as lymph : it fills all the chinks 

 o! the body and is in immediate relation to all the living protoplasm : 

 it forms the internal medium in which the various cells live. The lymph 

 is constantly being returned to the blood by definite channels known as 

 lymphatics, which open into the venous system at definite points, and 

 the walls of these may be in places thickened and muscular and form 

 rhythmically contractile lymph hearts but in the Dogfish the arrange- 

 ments of this lymphatic system have not up to the present been 

 rompletely mapped out. 



re leaving the vascular system the spleen must be mentioned. 

 This is in the Dogfish a large organ of a dark red colour fitted round 

 the bend of the stomach. It is really a kind of sponge-work, the meshes 

 ol \shich are filled with blood. Its precise functional significance is not 

 yet understood. 



The a- tiviiies ot the various organs of the vertebrate are controlled 

 and CO-ordinated through the agency of a very complicated nervous 

 in. 



'I'll-' <rnt ral nervous system consists fundamentally of a tube the 



neural tube with very thick walls and a very narrow cavity (central 



I), The greater part of the length of the tube is comparatively 



