The work presented in this thesis is related to the study of the longitudinal spin structure of the nucleon.
The aim is to determine the contribution to the spin 1/2 of the proton in terms of its constituents, quarks
and gluons. The analysis is performed on the data taken with the COMPASS experiment, which benefits
from a polarised muon beam at 200 GeV scattered off polarised protons from an ammonia target of 1.2
m long. The double longitudinal spin asymmetry of deep inelastic scattering cross-section. The spindependent
structure function of the proton gp
1 is derived from these measurements, which extend the
kinematic world coverage to unexplored region so far (0.0036 hxi 0.57, 1.03 hQ2i/(GeV/c)2 96
and 23 hW2i/(GeV/c)2 320).
The results obtained with a high statistical precision are included in a Next-to-Leading order QCD
analysis of world gp
1, gd1 and gn1 (proton, deuteron and neutron) data to parametrise the polarised quark
and gluon distributions. The g1 world coverage of the x and Q2 kinematic domain, which is a key
point in the sensitivity to the gluon polarisation G, turns out to be too limited for an accurate G
determination. Nevertheless, the QCD analysis allows to determine the quark spin contributions to the
proton spin to 0.26 < < 0.33 at hQ2i = 3 (GeV/c)2 in the MS scheme. The dominant uncertainty
on is related to the choice of functional forms assumed in the fit. Finally, the Bjorken sum rule,
which constitutes a fundamental test of QCD, is verified on the COMPASS data alone with a precision
of 9%.