326 ZOOLOGY FOR MEDICAL STUDENTS chap. 



combination 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. 

 Passing 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 

 arterial 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 

 oxygen : 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 

 food 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 

 of 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 

 completely mapped out. 



Before 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 

 of which are filled with blood. Its precise functional significance is not 

 yet understood. 



The activities of the various organs of the vertebrate are controlled 

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

 system. 



The central nervous system consists fundamentally of a tube — the 

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

 canal). The greater part of the length of the tube is comparatively 



