46 ZOOTOMY. 



last, the posterior face of which is devoid of them. Each 

 set of filaments is a demibranch or half-gill : each 

 partition with its pair of demibranchs constitutes a gill. 

 It will be seen that there are four complete gills, and one 

 odd half-gill forming the first of the series. 



XV. Make an incision through the sJiin of the head, 

 about an inch in front of the eyes, i.e. in the 

 region of the anterior fontanelle : lifting up the 

 edge of the incision, so as to see what you are 

 about, cut away the roof of the skull and the 

 overlying skin, until the brain and the anterior 

 part of the spinal cord are exposed. If no other 

 specimen is to be dissected, go over §§ 169 — 189; 

 then remove the brain by cutting through the 

 nerves and place it in strong methylated spirit. 

 If a special dissection of the nervous system is to 

 be made, remove to spirit at once,^ first noting 



85. The dura mater, or membrane lining the cerebro- 

 spinal cavity, in which the brain and spinal cord lie. 



86. The pia mater, a dehcate membrane investing 

 the brain and cord and containing numerous blood- 

 vessels. 



87. The arachnoid fluid contained in the cerebro- 

 spinal cavity. 



88. A mass of gelatinous tissue filling up the anterior part of the 

 cerebral cavity. 



1 For the satisfactory dissection of the brain, it is necessary for it to be 

 placed in spirit while fresh : it is therefore advisable for the student to 

 reach this stage of the dissection by the end of the first day's worli. For a 

 permanent preparation, the brain should be placed for 24 hours in a 

 saturated solution of zinc chloride, and then, after removal of the pia 

 mater, into strong alcohol, which should be changed at least once 

 (Giacomini's method) : the brain, either separate or in situ, may then 

 be prepared by the glycerine jelly process (p. 2, § I). 



