7i6 journal of Agriculture. [lo Dec, 1908^ 



THE ELEMENTS OF ANIMAL PHYSIOLOGY. 



W. A. Osborne, M.B., D.Sc., Professor of Physiology and Histology^. 

 Dean of the Faculty of Agriculture in the University of Melbourne. 



{Cciititiued from page 683). 



XVI. The Central Nervous System. 



The central nervous s\stem is composed of a vast number of neurons, 

 collected together and forming a column or cord with two pairs of speciaL 

 expansions. The greater part of this column is composed of the spinal 

 CORD which lies in a bony arch in the spine. This portion continued into- 

 the skull constitutes first the medulla oblongata, then the pons, thert 

 the region of the corpora quadrigemina, and then the region of the 

 THALAMUS. One paired expansion arises from the pons and is called the 

 CEREBELLUM, the Other paired expansion arises from the top of the column 

 and forms the cerebrum, or brain proper, a portion of the central nervous- 

 system which is relatively very small in the lower vertebrates but, in the 

 mammal, fills up the greater part of the cavity of the skull. That por- 

 tion of the column which lies within the skull, namely from the thalamus- 

 to the medulla inclusive, is frequently termed the brain-stem. 



A striking feature of the central nervous system is the large number 

 of nerve cells present which, when massed together, constitute with their 

 smaller processes what is termed grey matter. Intermingled with the- 

 nerve cells and their processes in the grey matter is a frame-work of cells 

 called neuroglia. The axons arising from the cells in the grey matter 

 when invested with insulating sheaths tend to run in strands and consti- 

 tute what is termed white matter. But the majority of the processes of 

 the nerve cells do not possess this sheath and do not extend beyond the 

 limits of the grey matter. The sheathed axons of the white matter 

 closely resemble the nerve fibres of the peripheral nervous system ; they 

 differ however from these in not possessing the external sheath or neuri- 

 lemma, and, probably owing to this deficiency, cannot regenerate when 

 injured. White matter is also devoid of the framework of connective 

 tissue which gives a characteristic toughness to nerve trunks ; in conse- 

 quence it is soft and pulpy and the constituent fibres are more easily 

 ruptured. In the chapter on nerve it was .stated that the cells of the 

 afferent and post-ganglionic neurons were all external to the central 

 iier\'Ous system whilst those of the motor and preganglionic neurons lav 

 within the central nervous system. But the.se latter constitute only ,; 

 minute fraction of the total number of cells in the grey matter. By far 

 the greater number of the neurons of the central nervous system are 

 connecting neurons allowing impulses to pass from one portion of the 

 C.N.S. to another and forming so many junctions, by means of synapses, 

 that each afferent neuron is placed in physiological connection with every 

 motor and preganglionic neuron and probablv with every cell in the whole 

 central nervous system. 



THE SPIXAI, CORD.— In this portion of the central nervous 

 system the grey matter forms a central core completely .surrounded with 

 white matter. The fibres in the white matter connect different regions 

 of the spinal cord with each other and further allow impulses to pass 

 from all its levels to the brain-.stem and brain and vice versa. One 

 very long tract of white matter carries impulses from the fore-brain to- 



