THE FROG 21 



the spinal cord. This is really a tube, but its walls are very thick 

 and the bore very small. If the neural canal be followed backwards 

 it will be found that the spinal cord is reduced to a mere thread 

 before it reaches the level of the hump in the back where the back- 

 bone ends. The reduced part passes back in the neural canal 

 accompanied by other similar threads, which also come off from the 

 cord. Sooner or later these pass outwards through the vertebral 

 column to form the nerves. At the anterior end the spinal cord is 

 continued into the head, where it lies in a bony case, the skull. 

 Here, however, it no longer remains simple, but swells out in various 

 parts to form a complex structure, the brain, which we shall have to 

 consider in detail later on. The whole of this soft structure, the 

 brain and spinal cord, is known as the central nervous system, and 

 it is the great controlling and co-ordinating organ of the body. It 

 is connected by nerves with the eyes, ears, nose, etc., whence it 

 receives messages from the outside world, and it is also connected 

 with the various muscles, and so is able to regulate the activities of 

 the animal as a whole. 



The frog will serve to illustrate the general plan of structure 

 common to those animals with backbones, i.e. Vertebrate animals. 

 An elongated cavity, situated partly in the skull and partly in the 

 vertebral column, extends along the head and trunk in the mid- 

 dorsal line, and contains the central nervous system. The nervous 

 system itself is hollow. The central canal, as the cavity is called, 

 in the spinal cord enlarges to form a series of spaces or ventricles 

 in the brain. On the ventral side, in the trunk only, is another 

 much larger cavity, the pleuro-peritoneal cavity, completely 

 separated from the neural cavity by parts of the backbone and also 

 by the dorsal muscles. A small portion of this is separated off to 

 form the pericardium, which is situated below the gut. A long 

 coiled tube, the alimentary canal, in which a number of different 

 parts are distinguishable, runs through the remaining portion of 

 this cavity, in which are situated the various structures which we 

 term the viscera. 



Before leaving the frog its blood should be examined under 

 the microscope. This may be done by putting a drop of it on a 

 clean slide and adding to it a drop of physiological salt solution 

 (i.e. '5 gram of salt dissolved in 100 c.c. of distilled water) to 

 prevent it clotting, and then covering it carefully with a clean cover- 

 slip. It will be found to consist of a fluid or plasma, in which 

 float an enormous number of small solid bodies or corpuscles. 

 Closer examination will show that these are not all alike, but are 

 of two different sorts. Some of them are oval, constant in form, 



