28o ANATOMY OF THE DOMESTIC FOWL 



The bodies of cells vary in size from 4 to 100 microns or more in 

 diameter. The largest cells occur in the inferior horn of the spinal 

 cord, in the spinal gangHa, in the large pyramidal cell layer of the 

 cerebral cortex, in the Purkinjean cell layer 'of the cerebellum, and 

 in Clark's column of the spinal cord. Very small cells occur in the 

 olfactory bulbs, in the granular layer of the cerebral and cerebellar 

 cortex, and in the gliosum cornuale of the cord. 



Nerve cells are classified according to the number of processes 

 arising from the cell body, and neurones are referred to as unipolar, 

 bipolar, and multipolar. 



Unipolar cells axe. met with frequently in early stages of em- 

 bryonic development, but are rare in adults, occurring only in the 

 retina, in the olfactory bulb, and within the baskets of the Pur- 

 kinjean cells of the cerebellum. The cells of the cerebro-spinal 

 gangha, except the cochlear and vestibular, are apparently uni- 

 polar, but they are developmentally and functionally of bipolar 

 nature. 



Bipolar cells are found almost exclusively in the peripheral sen- 

 sory system, as in the olfactory membrane, in the retina, in the coch- 

 lear and vestibular gangha, and in the cerebrospinal gangha of the 

 embryo. 



Midtipolar cells are the most numerous and form the principal 

 elements of nerve centers throughout the system. -They are termed 

 multipolar because of the greater or lesser number of dendrites 

 given off in addition to the single axone. 



The body of the nerve cell consists of a mass of protoplasm sur- 

 rounding a nucleus. The cytoplasm of the nerve cell consists of 

 two distinct substances: first, neurofibrils; second, perifibrillar sub- 

 stance. In most nerve cells there is a third substance called chro- 

 mophilic bodies. 



The neurofibrils, extremely dehcate, are continuous throughout 

 the cell body and all of its processes. Within the body of the cell 

 they cross and interlace and probably anastomose. 



The perifibrilfar substance is a fluid or a semifluid substance which 

 both in the ceU body and in the processes surrounds and separates 

 the neurofibrils. 



The chromophilic bodies are granules or groups of granules which 

 occur in the cytoplasm of all the larger and of many of the smaller 

 nerve cells. 



